Just spoken to somebody that worked at the Playboy from early 70s until it closed, they know nothing about a gold Playboy plaque just the Clermont one. He said that somebody took it away once but brought it back which was a shame as they'd hoped somebody would keep it.
A member of staff told me that it was inlaid into a blackjack table for a while but maybe a coffee table, I suppose.
The GM of the Clermont (who liked showing it off) said that it was sold when Mecca owned the casino, for £40,000.
So far that's the bulk of what I've managed to find out but there are still a couple of people I am trying to contact to see if they remember anything that can help to confirm, either way.
Obviously I will share any information I can find if relevant to either gold plaque.