Wasn't NIC/CYP previously identified as the work of an Australian(Sydney-based?) gunsmith?
There are loads of civvie-created No5s around, both No4 bodies milled to look like No5 bodies, and many where they didn't bother. At the moment, there is someone in UK having a good (yet so far unsuccessful) go at faking up electro-pencil markings..... caveat emptor....