The top gun shown above is an Improved I frame, but an early version with round sight; NOT the later version with ramp sight and barrel rib, which is the ONLY Improved I frame that can be confused with a New I frame: both have the smaller, rounder trigger guard.
.38/32 Terrier Improved I frames: EARY version on left, LATE on right:
Photo by jcelect
Below are .32 POST WAR HAND EJECTOR/REGULATION POLICE MODELS, but the frame changes are the same as the .38 S&W I frames.
You need to compare the TOP RIGHT Improved I frame (5 screw) LATE version (which as you see, came with the Type 2 flat latch) to: the 2ND FROM TOP on the right, a Model of 1953 NEW I frame (4 screw); ignore the barrel lengths and grip shapes on both).
The bottom gun on the left is an Improved I frame (5 screw) EARLY version.
LEFT SIDE:
.32 HE pre-war I frame Model 1903 (2nd Model)-5th Change, 3 ¼” produced Oct. 16, 1913, top;
.32 Regulation Police Transitional Post war I Frame w/Rd sight, 3 ¼”, 6 screw, ‘double pinch’ thumb piece, rebated sq butt, c. 1949, 2nd on left;
.32 HE Pure post war ‘early’ Improved I Frame w/coil spring, Rd sight, 2”, 5 screw, ‘single pinch’ thumb piece, rd butt, c. late 1951, bottom. This version of the I frame has the same features as the early round sight Baby J/Chiefs.
RIGHT SIDE:
.32 HE ‘late’ Improved I w/ramp sight & barrel rib, 3”, 5 screw, 2nd style flat latch, rd butt, shipped c. 1953, top right;
.32 R.P. Model of 1953 New I frame, 3”, 4 screw, 2nd style flat latch, sq butt, c. 1954, 2nd on right;
.32 R.P. Target Transitional I, 4”, 6 screw, 4th (final type) thumb piece, rebated sq butt, one of 196 from 1957 only, 3rd;
.32 R.P. J frame Model 31-1, 3”, 3 screw, sq butt, 1980, bottom.