If it is a .303 Regulated by Fultons it is nothing more than a regular No.4 that has been stripped and 'blueprinted' by experienced gunsmiths who knew what to look for and how to tweak it. There are probably some little hardwood chips in the stock bedding to firm up the bearing surfaces, and maybe some pads in the barrel channel.
If the barrel is stamped Ball Burnished, that means they dragged hard steel balls on long rods down the bore. This was a simple technique to iron out any roughness left over from the cutters.
There aren't many guys collecting high end No.4s, Wayne in Saskatchewan is one (and I can put you on to him). There is more interest in the 7.62 conversions for ammo reasons. Does it have a commercial style adjustable rear sight, or the standard issue one? Is it a MkI, MkI* or a later variant? Any accessories like a snap on front sight cover, a sling swivel in front of the magazine, or a 2" wide cotton sling with leather ends? All appropriate for the period.