Starting to drift off-topic here, but the improved roller pickup assembly has an 82xx part number, so that would suggest 1972 as the first year for the new solid roller design.
One interesting thing is that the revised roller assembly makes adding a 3-position E-unit somewhat difficult, compared to the 1970 vintage scout-type roller assembly. I believe you have to cut part of the plastic away to create room for the plunger. The only MPC four-wheel loco sold with a 3-position E-unit was the 8506 switcher, and to my eyes it looks like the plastic was cut away, maybe as part of the manufacturing process at Mt. Clemens.
That big stack of laminations, combined with a fairly tall gear ratio and rubber tires creates pretty high current draw on pulling and starting. The mid-70s version of this motor was wound with wire that IMO was undersized for the application (double-wound field.) They run hot, and sometimes get into a stall condition where the loco won't budge forward or reverse. This could be because the high temperature and current anneal the brush springs, causing them to lose tension. As a kid I can remember pulling my 2-4-2 off the track after a hard run and almost burning my fingers on the rollers. One of the projects on my bucket list is to repower a die-cast 8042 (or an 8204) with a single-wound field from the 520 boxcab, by transferring the wheels from the original loco. I'm pretty sure it would run slower and smoother, and the black side frames would make it look better too!
One more interesting tid-bit... Beginning in 1973 they offered a lighter-duty variation of this motor with a thinner stack of laminations. IMO that one runs smoother, the armature seems to be better balanced. But I've seen a few examples that have a problem with gear mesh. I'm not sure if this was caused by playwear, or if the tooling was starting to go out of tolerance after a few years' production. But when you can turn the wheels and the armature doesn't spin, that's a problem! 
