I just received it today and have no knowledge of its history other than it came from a pawn shop and appears to be completely original (except grips).
Tony,
That's a fine catch!!
People put way too much credence in the dates of changes and shipping. They're the least reliable of all S&W "trail markers". It's the features of each individual gun that's most important to identify what it is.
There's no mysteries about your gun, everything is kosher. The K22 Masterpieces were out selling the 4" models then. Your gun sat in the frame rack before being assembled a year or two later and shipped, hence it still has a 4 screw frame. It's completely normal for it to still be a 1961 Mod 18 pre dash especially with the lack of the engineering changes matching the lack of dash numbers. The ones that give us headaches are those that don't match; dash #s and no corresponding changes or changes with no corresponding dash #s. There's plenty of those around!
I have no doubt all parts are original. And the evidence is on the gun. The serial # is usually no longer on the barrel and cyl in that period (see below). HOWEVER all you have to do is check the back side of the extractor star because it's still stamped there. I bet it matches the gun # and your gun is 'golden'.
POST WAR SERIAL # LOCATIONS:
To confirm all parts are original, one can check for the 6 matching serial # locations on post war Hand Ejectors thru ~1956 and a few as much as 3 years later.
1. Gun butt - or fore strap on I frames/pre war single shots with grips that cover the butt
2. Barrel - bottom of barrel or in extractor shroud
3. Yoke - on rear face only visible thru a chamber with a flashlight (except the .32 Model 1896 and no doubt a few others)
4. Extractor star - backside
5. Cylinder - rear face
6. Right stock only - on back; stamped, scratched or penciled depending on vintage and stock material. (except most post war target grips because individual fitting not required.)
DECREASING SERIAL # LOCATIONS:
The number of serial # locations or if a Model/dash # is stamped on a particular S&W Hand Ejector has more to do with where it was in the production/assembly stages then when change orders were issued, therefore as we've learned to expect with S&W, there are great variances and exceptions galore.
Officially, on May 1, 1957 S&W eliminated the Soft Fitting Operation: So it generally corresponds with model numbers ordered June 12, 1957. It was no longer necessary to routinely stamp the serial number on the barrel, cylinder & yoke arm rear surface and show up unstamped over a transition period. So guns in process or in inventory as of 5/1/57 can still have more than 3 and up to 6 locations, and guns shipped after this time may have some of the former number locations because assembly was done over time and as inventory from the old process was used up.
The 6 serial # locations were down to only 3 left on the majority of models (but not all) from c. late 1957 to 1959 which are:
1. Butt
2. Extractor star - backside
3. Right stock – backside (except most post war target grips because individual fitting not required.)
ASSEMBLY (factory work) #s:
These multi-digit numbers of 3 to 5 digits, are on the yoke at the hinge, in the ‘yoke cut’ on frame (accompanied with a stamped inspector letter) opposite the yoke near the hinge, and inside of the sideplate, for the pre war and early post war period. Once the gun is shipped, the only use for the assembly # is to confirm the three parts it's stamped on are original.
In 1957 the assembly # in the yoke cut of the frame was relocated to the left side of grip frame after model #s were assigned and the serial # was eventually added in the ‘yoke cut’ where the assembly #, now moved to the left side of the grip frame, used to be. You know they are assembly (factory work) #s because of those 3 locations that always match on guns that are original, and that’s the only usefulness for them after guns leave the factory; still used to this day, long after serial number locations decreased.
MODEL NUMBER STAMPING:
began eventually, sometimes months after being ordered June 12, 1957, but the serial number was not yet stamped in the ‘yoke cut’ of the frame.
YOKE CUT STAMPING of the assembly # vs. serial #:
changes after Model # stamping got more confusing. When Model Number stamping began some months after being ordered June 12, 1957, the serial number was not yet stamped in the frame ‘yoke cut’ on the frame side of the yoke hinge. But in the l958-1959 time period as the use of target grips became more common and if the revolver was ordered with or was standard with target stocks which covered the butt serial #, the serial number is also stamped in the yoke cut of the frame. The assembly number would also be in this area as it was stamped on the gun long before the revolver was serial numbered.
But soon after, all models have the serial # stamped on the frame in the 'yoke cut', except on the smaller frames at first where it just didn't fit well. So eventually after the serial # was stamped on the frame in the ‘yoke cut’, the frame assembly # from the 'yoke cut' was moved to the left side of the grip frame. The 3 assembly # locations, on yoke, left side of grip frame and backside of side plate remain to this day. However, the post 1959-60 yoke cut serial # location on these later guns is the reason for many pre 1957 guns to be incorrectly registered by the useless assembly (work) # on the frame in the yoke cut.
Therefore by about the end of 1959, serial # locations went up to 4:
1. Butt
2. Frame in yoke cut
3. Extractor star until ~ 1980 when the new extractor star shape was introduced.
4. Back of right stock, until ~ 1979 when no longer hand fitted, (except most post war target grips because individual fitting not required).
EXTRACTOR STAR Serial #:
“…were numbered from the early Hand Ejector models through about 1980. They were numbered because the ratchet lugs had been fitted to each particular gun’s lockwork, and it would be separated from the gun at some points of finishing. This is true for blue, nickel, and stainless guns.
"I've observed a lot more since I made that statement. Oddly, I have often observed all original 4 Screw K and N frames with numbered cylinders when none of the other parts were numbered, including the extractor.
“Obviously, something had changed in the fitting and/or finishing sequence for a short time that did not require numbering the extractor.
“Apparently, that sequence was changed again, and we arrive at the long used process of serial numbers on the frame and extractor only.” Regards, Lee Jarrett
Once the new shaped extractor star is introduced apparently beginning c. early 1980’s, and completed on all models by c. mid 1990s, it seems to have been coincident with deleting serial # stamping. Only the butt and yoke cut stamping locations remain to this day. But again with exceptions like anything S&W, as we some later introductions with laser engraved serial #s in new locations.
For reference, my 4-Screw Model marked 18 (with no dash) is in the same range as yours (SN K433575) and although I have not ever dated it, I always believed it to be shipped in 1961. Here are a few terrible photos of my gun. 
