OK, after doing some research I think I have answered my own question. After reviewing the master parts catalog ('29-'57) and a great number of eBay auctions, I have discovered:

both years use a drive shaft OD (rear bushing ID) of 1-1/16"
both years use a yoke diameter (front bushing ID) of 1-11/32"

The difference IS the torque tube ID:

40-50 (1st design) has tube ID (front and rear bushing OD) of 1-11/16" = 1.6875"
51-54 (2nd design) has tube ID (front and rear bushing OD) of 1-49/64" = 1.7656"

So, if this is correct, the difference in torque tube internal diameter is a very small but important .078" (about two spark-plug gaps). SO, no wonder my 40-50 okie bushing looked like it might fit in my '53- because it ALMOST will.

Thanks for looking, and if I got anything wrong above please let me know. -Warren