Interesting that the two differ in their listings, although with provision of the ALCO-GE manual data we can hardly argue the model designations.
A quick look at the manual shows that the diagrams depict the steam generators and associated equipment, with the "B" unit having the steam generator mounted in what would otherwise have been the cab end, or the "A" end in the same manner as was done with the larger 2000 or 2250 HP passenger locomotives. I noted also the arrangement of the fuel and water tanks, and a quick look at the net reveals a couple interesting photos.
http://www.rr-fallenflags.org/bo/bo803amm.jpg
The above link, from George Elwood's fabulous site shows an A-B-A lashup of B&O units. The unit on the left does not have a steam generator while the other two do. Note the difference in the underbody tanks and the roof appurtenances. Note roof vents behind radiator on the far end A unit, and note roof vents on the opposite end of the B unit in comparison.
http://www.rr-fallenflags.org/lv/lv594as.jpg
The above, same site, is obviously an LV FPA-2. Again note roof appurtenances and the large fuel / water tank. The row of bolts along the bottom is indicative of the dual tanks if photo perspective makes the actual tank dimensions unclear, as in for example in a more head-on or wedge shot.
I looked quickly at a Don Ball book that shows the 1603 set of ALCO-GE demonstrators on the Western Maryland, and they had no steam generators so I looked online and found the 1602 set which apparently does, given the appearance of the underbody tanks, at Northeast Railfan:
http://www.northeast.railfan.net/images/alco1602A.jpg
So then, photographically we can fairly easily figure out what units had, or did not have steam generators even if we can't see the roof and given the ALCO-GE manual we can be sure they're FPA/FPB models officially.
-Will Davis
PS I'm sure many members here know the detail differences, but I thought I'd include all of that for those not up to speed or who didn't know how to tell the difference. Excuse my long-windedness!!