It's factory.
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. Not to mention pinching a fingertip or two.
The cylinder hold open device or detent was introduced on the very 1st hand ejector, the ".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.
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 surplus Model 1917 yokes being used from inventory, they are still found in most N frames pre WWII, and even thru some post war Transitional N frame HEs.
Another example of old world panache Smith was known for that we won't ever see again. And a very useful, easy to add device to any later model Hand Ejector.
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 it's missing, that's likely what happened to it.
Also they were never installed in any "cranes", S&W Hand Ejectors only.
Plunger shown here in bottom of yoke and has a spring underneath it:
Photo credit: handejector