OK, my question was prompted by the acquisition of this 30-1, you see. It does indeed have a round butt (and 3" standard barrel.) SCSW says the 30 came with RB or SB. Available barrel lengths are pretty much the same, and IIRC, there are some RB 31s out there. In short, it appears that the model designation depended on which stamp the final assembler had in his hand! Of course the 30 was the successor to the 32 HE and the 31 continued for the .32 RP, but once the barrel and grip differences began to overlap, those distinctions disappeared.
I guess the reason for the seeming dearth of 30s compared to 31s is a function of the length of production run(?) Ultimately, I guess we can say that they really are a single model with minor variations within as well as between model numbers.
Froggie