Some years ago I discovered the inserts by accident while reading up on S&Ws:
The Model 1896 .32 HE, the 1st side swing S&W revolver, first made use of hardened steel shims to reinforce the cylinder stop notches. According to Jinks' A History of Smith & Wesson, S&W installed hardened steel shims from 1896 until 1908. The practice stopped in 1908 because S&W began using an improved steel for cylinders. ***
Measuring their thickness on my Mod 1896 with calipers and 10X glasses I read .025" for the full length of the notch:
Photos by daivedj
*** At the tine, further research revealed that apparently both .32-20 and .38 HEs did indeed also use cyl notch shims; all 32-20s but only commercial (non-military) model 38s, S&W 1857 -1945, N&J.
This from N&J (Revised Edition).
And-------32-20 HE First Model---1899-1902, #'s 1-5311: "The cylinder stop notches were lined with hardened steel shims to prevent upsetting."
1905 Second Change----33501-45200----------and "The use of hardened steel shims in the cylinder stop notches was discontinued late in the manufacture of this model."---sometime prior to 1909.
The 38 HE 2nd change includes the same notation.
There's also a statement in the 32 HE Model of 1903 2nd change 1906-1909 section indicating the shims were eliminated near the end of production for this model.
In Jinks' A History of Smith & Wesson, the practice stopped in 1908 on all three models because S&W began using an improved steel for cylinders.