Hello Changling. Pre describes a time before a certain event or change.
"Examples model 29 or 27 : When someone says this revolver is a pre 27 or 29 it confuses me, how does one know it is as it says"? Using either as an example the pre model 27/29 will not be marked on the frame, one can open the cylinder and marked on the frame are frame numbers that do not relate to serial number nor model. If the gun is not marked MOD 27/29, a 27 might read 27-2 meaning this model has had modifications and is no longer a MOD 27-1.
This link gives some good descriptions and history of the MOD 27.
m27
"How do you tell if it is what is represented and how do you tell if it is a pre 27 is the same". If a Model of 1950 in 357 Magnum is listed as a pre it will not have MOD 27 and or MOD 27-1 or -2 or -3 etc stamped on the frame.
There are lots of pictures on this Forum, search pre 27's or 27- 1,2,3 and you can see the measurable difference is the stamp or lack of a stamp on the frame. The intrinsic difference is collectability, perceptions in quality, FINISH, accessories like the box and tools. Usually age represents more money with condition being king.
As several have mentioned the pre guns had romantic or wonderfully descriptive names, like Combat Magnum, I suppose it was more of a corporate decision to make their life easier. This revolver is a S&W Model 27 revision or dash 2. This gives them precise numbers to track and left us asking again, what is a pre model something?