Hmmmm -- does that mean the early boxes were not designed correctly?
They were probably designed correctly (it's not exactly difficult to spec the working voltage of the caps, after all!) but not assembled correctly. Or failing that, the specced caps weren't available and the 16v ones (which would be right on the edge of a +/- 15v spec) were fitted instead. Normally that would actually be okay, but if there's anything even
vaguely dodgy about the caps themselves, then their lives might well have been shortened.
One way or another, it's a bit moot, really.