The last time I spoke with Great Plains Audio, they told me the only difference between the 416-Alnico and the 515-Alnico was the magnet size and strength. I asked about the cone and surround and they said it was exactly the same for both drivers.
Now, back in the vintage days of 1973 and earlier, the 515 could well have been different from the 416. The problem with the vintage, Altec-made drivers is they changed them every few years, so there is no "one" 515 or "one" 416.
Prior to 1973, before Theile and Small appeared in the AES Journal, nobody measured Qts, Vas, or Mms. If the production people wanted to "improve" the driver or cut costs, they just went ahead and did it. As a result, vintage 515's and 416's are all over the place, and not necessarily comparable to current GPA production.
I know this is true for Altec, but I'm pretty sure it's also true for vintage JBL, Jensen, Electro-Voice, or Stephens Tru-Sonic. Before T/S, there were vague theories and rules-of-thumb, but not the precise mathematical modeling T/S made possible.
One of the big breakthroughs in those AES Journal articles were accurate measurement techniques for the critical parameters of Qts, Mms, and Vas. In the first years of T/S, there was a frenzy of measuring every driver they could get their hands on, since no manufacturer specified these parameters ... they didn't know them themselves. It took many years for T/S parameters to be routinely published ... several manufacturers were quite secretive, and this was long before the Internet. You had to know somebody, or take a class and look at the data they had collected on their own.
In 1975, I took a class from Bob Ashley, then head of the AES, about the ins and outs of measuring T/S parameters, and saw a IBM fanfold printout of 12 different Altec 15-inch drivers, all with quite different parameters ... notably, with different Mms as well as different Qts. Altec was all over the place in 1975, and presumably earlier as well.