I picked up the serial number in the SCSW 3rd edition while researching the model 645. I'd never heard about a 10mm 645 before. Never come across a photo either.

An old collector told me about this pistol and the "Supergun". The Supergun was an early S&W 45 prototype that resembled a black 645.

IIRC Mas Ayoob may have been shown the 10mm prototype during a visit to the FBI back in the day. I'll see if I can confirm that. Regards 18DAI.