I recently read something that desribed the introduction of the PowerBook 5300 and the negative impact it had, and that Apple produced the PPC upgrade for the 500 series as a response so users could have a well-built notebook with a PPC processor. The PPC 500 series was basically a stop-gap before the release of the PowerBook 1400.

The upgrade cards themselves are very rare and probably mostly lurk inside long-forgotten 500 series PowerBooks. I'm still blown away by how cheaply you got it!