Provenance. Unless that PCB has a certificate of authenticity CoA witnessed by Woz or Apple - maybe echoed as a smart contract on the blockchain - it's just another busted PCB from the 1970s (plenty of these pictured on this forum). Without immutable provenance, I would value the incomplete artefact in the $1500-$2000 range. About the same as a fully functional first version CBM Pet or Pacman machine.
A collector has no interest in what it is but, what it will be worth in 10, 20, 50 year's time? Especially if they are a private equity fund investor. The value is the history, the story, the importance, the provenance. And the only expert appraiser is the soldersucking man himself.
Buyer beware.