The key element so far appear to be:
1.) The treadle wheel position - Mine is below the deck, the Model B is "peeking out above"
2.) The bobbin winder - Mine is not equipped with one and no mounting points are evident.
3.) The tensioner - all of the Model A drawings and pictures so far show a "corrugated" tensioner disk, mine is brass and smooth
4.) The tensioner - the later models have a little extra wheel on the side closest to the spool holder mine does not.