I don't know whether, as a matter of law, the invisibility of proof marks renders a firearm out of proof; however, evidentially-speaking, their absence will be a major obstacle to a lawful sale.It appears to be the case...
The question is, is it out of proof since there are no "visible" proof marks?
If the chap wanted to sell it, would he have to send it to the proof house prior to sale?
I know that it's an offence to alter or modify a proof mark , or to counterfeit a proof mark, but I'm not so sure about removing one such as by shortening a barrel. The marking of a barrel near the crown is only something that the proof house have done in recent years and many would argue that that was done as a matter of protectionism to justify their take on the situation regarding threading.It is I believe an offence to remove proof marks. For example the Proof Houses now stamp the muzzle. So if you thread or shorten the barrel you have to reproof because you remove a mark in the process