Differentiating I from J frame:
I Frame - 5 screw frame, short cylinder (1.25- 1.26), strain screw on frame
Improved I Fame - 4 or 3 screw frame, short cylinder, no strain screw
J Frame - 5, 4 or 3 screw frame, longer cylinder (1.525), no strain screw
The new design hammer block was instituted in post war production at S/N 536685.
Thanks for that info.
What's a "strain screw"?
DF