While perhaps not definitive, the serial number list on Selmer's website seems like it would be the best source. Seems like the only time there might be a mix of the two would be during the published first year of production of the SBAs (like 1948 or late 35,XXX serial numbers). Another characteristic I've noticed is that later SBAs had a single key guard for the lower B and Bb keys (as with all mark VIs and later models) whereas it seems all BAs and some early SBAs had separate key guards for low B and Bb. Not very diffinitive, but something I've noticed. Having just tried a BA and an SBA, they were very similar as far as tone. Very sweet. I prefer my early Mark VI personally as I can get a tone that is just as sweet, but it is easier to get a more aggressive sound. Just more flexible really. Also, the comfort of the action on the Mark VI is quite an upgrade and allows for much more fluid speed. Just little things like the comfort of the left hand thumb rest and the octave mechanism. I know I'm am too spoiled by it to really enjoy playing a lot of other vintage horns, so my search has been over for years.