Attached is a National Malleable and Steel Castings Co. ad from the Aug. 30, 1941 Railway Age, showing both their B and B-1 trucks.  A noticeable difference between the two are the bolster ends.  Also the springs on the earlier  B truck were larger diameter than usual and as the capacity increased so did their diameters, but the wheelbase stayed the same.

P2K/Walthers offers the B truck, while Athearn and Kadee sell the B-1 truck.

This afternoon I tried downloading this scan into both Files and Photos, but neither would accept new additions.  If interest in seeing it, just contact me off-list at brianleppert at att dot net.