Over the years there has been quite a bit of discussion among SPF's (slobbering Pennsy fans) about variations among PRR N6b cabin cars. In addition to the somewhat similar "wide cupola" N6A, there were at least 3 prototype versions of PRR N6b cabin cars. The easiest to spot was the centered cupola version. Less apparent were differences between offset cupola versions. In the most common version the side windows were centered over the trucks (as found on Precision Scale and Quality Craft O scale models). This version is best represented by restored N6b 980016 illustrated on the cover of The Keystone Vol 22 - No.2. However on a few N6b's the car body windows were located closer to the end walls, creating a different visual relationship of the windows and the cupola. This is the one Max Gray imported from Japan in 1963 priced at $14.50. (illustrated in Bob2's third photo posted above). Variations in prototype N6b's apparently occurred as some cars were built new, while others came out of different PRR shops using body sections of obsolete wood underframe 4 wheel bobbers that were mounted (and extended) on steel underframes.
For the post war O scale 2 rail modeler IMO the best detailed N6b's were the ones imported by Precision Scale in offset and center cupola versions. I particularly like the high arch single spring cabin car truck PSC chose to create for the model, and the underbody detail. Note that the PSC models (and MTH) N6b's) have a second horizontal end railing found on postwar N6b's, while the Max Gray N6b's have the as built single horizontal end railing.