I think this alludes to another factor in play here. Chain is supplied in very few sizes compared with the number of saws it can be run on. Consequently, the same chain can be run on saws differing widely in power, which can be developed through different combinations of torque and revs.
Take, for example, an 051 and an 064, which are pretty similar in .cc (I know the 051 was usually fitted with .404" rather than 3/8", but assume for argument's sake that it's been switched over). These would be similarly rated saws, and would take the same chain - full chisel or semi-chisel, your preference. However, the 051 is low revs, high torque while, relatively speaking, the 064 is high revs, low torque.
The chain out of the box has to be a compromise that will perform acceptably with both. By definition, this means the chain will need some alteration to optimise it for the saw it's been fitted to. For a '70s/80s saw I would expect lower rakers to take advantage of the torque and drag out big chips, which will allow them to keep up with a higher revs saw in cutting performance if you take into account that the rakers will need to be set higher on the latter.
The same chain will also go on the 036. 038, 044, 046 and 066 (or whatever the latest MS equivalents are) which spans a huge range of power output. Again, you will not get as much out of a big saw if the chain has been optimised for the smaller one, and you will stall the smaller saw if you optimise the chain for the big one.
Most of my experience of the above is from rip cutting (milling) but here you see the differences very clearly as the cuts tend to be wide, so they're using most of the available power, and you make long, steady cuts so you get a good sense of what the saw is doing, rather than stop-start work where it's harder to work out.
Alec