CYL/YOKE DETENT HOLD OPEN DEVICE:
A classic old feature to keep the cyl open when loading/unloading and prevent it from slamming closed if the gun is tipped to the right slightly and scratching the recoil shield with the extractor star.
The cylinder hold open device or detent was introduced on the very 1st hand ejector, the ‘I’ frame ".32 HE Model 1896, 1st Model" thru the “.32 HE Model of 1903 – 2nd Model”, thru ~1904.
It was used and continued in pre WWI K frames beginning on the “38 M&P Model of 1899” thru at least the “38 M&P Model of 1902 – 1st Change”.
And also the N frames beginning with the 1908 44 Triple Lock.
It’s unknown if the M frame had the device and highly unlikely that it did. But if it did, it would be the 1st and 2nd models, possibly early 3rd models.
By the Mar 10, 1919 change order: all cyl hold open detents in all frame sizes had been ordered eliminated. However, likely due to many N frame yokes/frames in inventory accommodating the device being used from inventory, following heavy production for the .44 Triple Lock, .455 British Svc Revolver, and later surplus Model 1917 parts which all did have the device, they are still found in most if not all Pre WWII and even Post War Transitional N frames.
Another example of old world panache Smith was known for that we won't ever see again. But very easy to add to any later model Hand Ejector.
Replace parts in old gun or Install in new gun:
To install a detent in a gun that doesn’t have one, drill the vertical hole in the bottom of the yoke hinge with a #37 drill bit to a depth of .890", for the K and N frame, and cut .540” to .570” (for K and N frames) off the shank of a #38 drill bit (.100”-.101” diam.) which fits the drilled hole in the yoke. The yoke is hardened somewhat so plan on using a cobalt or carbide bit to drill a nice clean hole.
Use a BIC lighter spring cut to about ˝”. Install spring and pin with domed end out. You can experiment with different spring lengths for just the right tension. If the yoke pivots too tightly cut a coil or two off the spring. Keep in mind that none of the dimensions are critical.
Use a sharpie pen to blacken the frame side of the yoke hinge. With spring and plunger installed, install the yoke/cyl assembly in the full open position. Close cyl, reopen and then remove yoke/cyl assembly. Center punch and drill the dimple shown below at the end of the inscribed line made by the detent pin in the black ink.
Important Note: when you remove the yoke beware of the cyl detent pin and spring! If you aren't careful, it can launch across the room to no-man-knows-where! If the plunger and/or spring are missing, that's likely what happened to them.
Plunger shown here in bottom of yoke and has a spring underneath it. The yellow rectangle indicates the plunger travel mark which indicates if the gun had the device or just the dimple on the frame and hole in the yoke w/o parts installed at the factory:
Photo credit: handejector