Bought a like new p220 today ,,Slide says on the side " Don't tread on me"on top it says "1 of 50" in gold lettering ,,Can't find any info on it ,,Is it rare? Should I just shoot it or put it up ? any suggestions?

It's likely a commemorative buy, no special value unless you find someone who has to have it.

I found some references of the P226 and M400 being embossed with that, but did not find anything on the P220.

Maybe if you contact SIG customer service they could provide more information.