Looks like a normal No4 Mk2.

By the colour of the wood, probably in the mid-range of PF numbers. Is there the remains of a number on the underside of the forend at the front?

It does appear to have been repainted, so its possible that the receiver has been skimmed or polished before it was refinished. Possibly done to remove military markings for some reason - or hide its provenance! The "666" does look like a rack or ownership number, possibly from a small national arsenal - ie maybe one of the Caribbean islands or other small former colonial territory.