The few folding stock shotguns that I've used all had "law enforcement only" stamped all over the stock extensions. I think that was part of the manufacturing process in the USA. So no idea about actual law I'm afraid.
You can have one on a shot gun cert but you would have to have the mag capacity restricted to 3 rds and have a barrel length of 24 inches. If more than 3 rounds you can still have it but it has to go on a Fire Arms Certificate which can be a bit harder to get.
Sorry MSR, the SPAS 12 with the extension taking the bbl up to 24" (which was of course permanently fitted to the bbl prior to the wpn leaving the factory,) only measures 34-3/4" so unless you've got a friendly neighbourhood Home Secretary who'll sign you off a Section 5 Cert you're clean out of luck.