I never saw either till the '80s so I can't speak with authority, but I believe both, when introduced as the DF series, shared the doomed cloth half-roll, which has a very similar cross-section as the later foam. By around '75, the 413 was done for, and the 411 got foam for the remainder of production. The rogue appearances of accordians were aftermarket refits, regardless of the semi-recent troll post claiming otherwise. GPAs' are foam, and indestinguishable from older Altecs.
AFAIK, the same foam from GPA for 411s should work on 413s, and if so (easy to tell with a measurement and a phone call) would be a recommended refit. The 4 411s I've reconed all suffered from serious spider fatigue, but that issue seems limited to the 4" offset spiders.
As to the developement issue, my speculation is the 411 outperformed the 413 enough that they saw no reason to keep the 413. As to production cost, Altec wasn't making their own software anyway, so ordering different parts to spec is just that. For the frames, I've never noticed a difference. The std frame opening for the coil and spider land is sufficietly large enough to accept a 4" VC, only the front plate and pole would need to differ.
...grain of salt...
Added...
For the OP, if unhappy with the 413s, just spring for some new 3154s from Bill and be done with it...
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