There won't be any paperwork to say it's deactivated...as technically it was never active. It has never been through proof. There will be no proof stamps for anything, including deactivation. I'm assuming it is a 100% PH inert unit. I've never taken my inert ones out of the stocks to look in detail about plugs.
I've read that when PH went defunct (or at least stopped making the Enfield repros) a number of components were sold to an Italian manufacturer and these were turned into guns of hybrid-manufacture for a while, but I think the Italian ones were only live - ie not inert.
So don't panic that there is no documentation to say it is a deact. This is treated no differently to, for example, the inert Indian guns from Henry Krank. These are freely sold legally off-ticket and are made on the same production lines as proofed live guns. They turn up at fairs and sites like Arundel Militaria with some frequency.
I've some PH sales lit from the time that I'll try and scan in when I'm next in the office (as I've bust my scanner at home).
You are committing no crime by possessing it as inert. If you want to try to convert it live then that's up to you, the proof house and your FEO...