I’m guessing that this is a post-war pen. Though the beautiful hatched celluloid (which almost every company used) was around in the thirties, it became much more commonly employed after the war. The flat clip looks like an early fifties design to me, as do the closely set medium/narrow/medium cap rings.
At first glance I took this to be the quite common arrangement of a single broad cap ring incised and painted to look like three, but no, this is the real thing, as the picture shows. The use of aluminium in the clip and (quite possibly) the cap rings seems like a late development for the company too.
Though they made many lever fillers and even a bulb filler (so I’m told) Mentmore excelled at making very efficient button fillers. The usual brass button has here been replaced by a screw-in plastic one which was novel enough when this pen was made to require a patent of its own, as is recorded in tiny embossed printing on the end of the button.
It is often the case that things become most dear to us when we’re on the point of losing them. If my dating is right, and this is an early fifties pen, the British Empire was well down the long slide to dissolution as emerging nations booted us out left, right and centre, but the company chose, perhaps defiantly, to call this “The Mentmore Imperial”.
Though Mentmore was soon about to be quietly allowed to die while Platignum took over, this pen shows no sign of the company’s imminent decline. The substitution of base metal for gold plating in the trim may well be an aesthetic decision rather than a cost-saving. The cap and barrel are machined from the rod, instead of being made from wrapped celluloid, a saving that many companies were making at this period. Everything fits together beautifully and the well-machined threads remain unworn today, even on the blind cap which gets used frequently. Mentmore were still making fine writing instruments at this point.