From what I can deduce from my copy of Perazzi Shotguns by Karl Lippard there are many models with flat ribs, and some have the "B". However, not all Perazzi guns with flat ribs have the "B" designation. As I look through Chapter 2, entitled Perazzi models, I see that the there was a Granditalia, an MS80, a MX3B, an MX4, MX7, and an MX12, all of which are listed with flat ribs.
The MX28 and MX410 both are described as having a Very Small Frame, but the book does not specify that they had flat ribs, and no mention is made of a "B" in the model designations.
There was also a Small Frame SCO Field Gun, with no mention of flat ribs. The serial numbers on these models are 10,001 through 10,050. They were made from 1979-1984.
While Lippard's book is THE Perazzi book, I suspect it is not 100% comprehensive. Also, just because I answered doesn't mean I'm a Perazzi expert, by any stretch. I just happened to have bought the book years ago, to help get some details on my MX8.
