I am curious, are these SB and Ar-2 profiles for publishing or it's your personal interest? Either way you have to read Maslov's book. It is explained there that shape of engine cowling does NOT relate to particular engine type
. Designations like "SB 2M-100A" or "SB 2M-103" are almost useless
. The same can be said about everything that was written about SB variants before Maslov's book - what was previously identified as SB M-100A were likely planes with M-103 engines and frontal radiator etc, etc.
If you want to accurately describe SB variants you should use number of a Series, similar to LaGG-3 Series. The problem is that Maslov (or any other author...) haven't so far identified several distinctive modifications by the series number. So, good luck...
This particular SB is one of least known modifications - "SB dive bomber"
. Those were made by Zavod 22 before it started producing Ar-2.
Some of the SBP characteristics:
- it was equipped with bomb cradle like Ar-2
- it had fighter type gun sight
- pilot's seat was raised so that he could see target over the plane's nose
- cockpit canopy was domed to accommodate raised seat
- "LU" ventral gun similar to one developed for Il-4
- MV-3 dorsal turret
- lowered (retractable?) back behind MV-3