No, the 500 series was marketed as ppc-upgradeable. So were other late 68k powerbooks like the 280c. It was standard marketing fare c. 1993-94.

There were several 603e ppc upgrades for the 500 series, at least one of which (Newertech) ran at 117MHz, which I recall because I have one. Apple produced a slower (100MHz?) upgrade card, and Newertech a variety at even higher speeds, up to somewhere just short of 200MHz, I believe. I suspect that these latter cards would be exceedingly rare, as by the time of production, the 1400 series must have been available.