They aren't that scarce. No import stamps usually means that the rifle was surplused prior to the 1968 Gun Control Act. In the case of the No.4, your rifle may well have been culled by the military because something is out of specification. I hate to burst bubbles here but this is a fact although there were some good rifles surplused too prior to 1968. The No.4 was kept in reserve stores until the 1980's in the UK and other Commonwealth countries. The best ones I've seen, (Britishrifles), were surplused directly from Donnington in the late 1980's and arrived in the USA
via Interarms in the early 90's. Then the South African reserve stores which were also imported via Interarms. I have a 1942 Maltby just like yours and have seen many others. I have two 1941 Maltby rifles that are also excellent but I'd never advertise them as "mint" or "unfired". It just ain't realistic. This horse has been flogged to death in my humble opinion.