Originally Posted by Savuti
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