Almost correct.

Models 30 and 31 are .32 caliber guns; Models 32 and 33 are chambered in .38 S&W.

The model 30 is the numbered descendant of the .32 Hand Ejector (round butt).

The model 31 is the numbered descendant of the .32 Regulation Police (square butt).

The model 32 is the numbered descendant of the .38/32 Terrier (five shot, round butt, short barrel).

The model 33 is the numbered descendant of the .38 Regulation Police (five shot, square butt).

I don't know enough about the model-marked guns to say that those classifications are rigid, but I know in the postwar period before model numbers were introduced there was some confusion between characteristics of these guns. I have a transitional .38 RP with a round butt, for example. I believe there are .32 revolvers that could be described as either Pre-30 or Pre-31 depending on which aspects of the model definition you emphasize.

Maybe somebody has a rigid typology worked out and understands these guns, but in my mind the lines between pre-30 and pre-31 can blur a bit, as they do between pre-32 and pre-33. If you have a box for a transitional gun, you can at least be guided by the label. After model numbers were introduced, the gun itself should tell you what it is.

Yes, these models all started before WWII as I-frames, and then morphed into J-frames after the war.

Only .38s carried the "Terrier" name; there were no .32 Terriers.

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David Wilson