There is a "StuG IIIa" and a "StuG IIIb" because, in reality, there were "StuG IIIa"s and "StuG IIIb"s and for all game purposes they ARE identical . This is, I am nearly 100% sure, the result of someone in the past wanting to add more "realism" for "completness" to the OOB.
I fully understand the above. Why then in OOB v5.0 the StuG IIIb has an odd SMG? (unit 035 / oob 16)
That unit was altered like that over 4 years ago because, as I recall, it was shown to be common practice to use the SMG for close self defense in some units so you have the StuG IIIa with only the main armament or the StuG IIIb coving pretty much the same time period with the SMG