It's a 32 Hand Ejector Model of 1953 New I frame, the last iteration of the I frame design. The barrel is 3".
You're correct, it's a Pre model 30 if it has a rd butt, most likely shipped in the first half of 1958 just before they were stamped Model 30. If it has a sq butt, it's a 32 Regulation Police, Pre model 31.
No difference from the Models 30 and 31, but the Models 30-1 and 31-1 are made on the J frame beginning Jan 1961.
In 1966 the standard thumb piece replaced the flat latch and in 1968 the large diamond around the grip screw was deleted.
If the nickel finish is original, it's worth in the $500 ball park w/o box and tools. But if the hammer and trigger are not color cased hardened, the nickel plating is not original.
There are two I frame and two J frame grip sizes. I and J frames and stocks are the same size for the same vintage.
1. Before 1953 and with 5 or more screws (back to 1896 for the I frame), the I and J frames and stocks are the same size.
'Mini' diamond Magnas were used on I and J frames after the demise of the 'rd top' Service grips of pre war style.
Pre war I, post war I, and Improved I grip frames (and Single Shots), and the Baby J grip frame up to 1953 are the same size and interchangeable, whether 'mini' Magnas or the 'rd top' Service grips of pre war style.
2. The Models of 1953 New I frame and New J frame grip frames and stocks to the present with 4 or less screws (until the demise of the NEW I frame Jan 1961) are also the same size, both 1/8" longer than the pre 1953 RB stocks.
The 1953 I/J square butt grip frame never had square corners like K/N frames; they have very rounded corners.
Much more detail here in the commentaries:
THE POST WAR I FRAMES EVOLUTION
http://smith-wessonforum.com/blog.php?u=134353
