I’m not sure that the TSB actually clarifies the question. It’s referring to the track width change between the 67-72 drum brakes and the ‘73+ disk brakes.
That change is well know, going from the 67-72 SBP drums to the 73+ disks results in a pretty large change as described in the TSB. But the TSB does not list different track widths for the 73+ cars, it only lists one. Of course, it only lists one track with for the 67-72 cars too, and I don’t believe the 67-72 drum and disk cars had exactly the same track width either. Although the 73+ disk track width is wider than both the drum and disk 67-72 cars.
