We do know that the black powder itself was once more potent. Our esteemed Estes folks have talked about the formulation and volume needed back then vs volume needed now to attain the same total impulse. As long as they stayed on top of the changes in BP and tweaked the volume (I doubt they fooled with nozzle tweaks), they would stay in the ballpark in performance over the decades.

If a weaker formulation was compensated with greater volume thrown into the casings and no nozzle changes were made, we could end up with a weaker max thrust but total impulse very similar. That would probably give us minimal altitude variation on light rockets. It would explain decades of seat-of-the-pants performance testing where we noticed C6-5's of the distant past doing better with heavier rockets than today, while your tests proved that total impulse is likely very close to motors of decades gone by.

Speculation, of course.

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