Usually the company made a clear distinction between Mentmores and Platignums but this pen is the exception. Everything about it, the pattern and the feel of the material, the glittery chrome-plated trim and the tiny nib all say Platignum, but it’s a Mentmore. It has neither model name nor number.
It’s a button filler, 13 cm long. The nib is warranted and so may not be original though I suspect that it is because it fits so well. The nib has considerable flex.
This pattern and style among Platignums dates to the late thirties and doubtless it’s the same when it has a “Mentmore” stamp on the barrel. The material of barrel and cap is quite thick, making for a very robust pen.
Goodwriterspens's Blog 
thanks for posting Deb. For the various reasons you mention, it’s an interesting example of a Mentmore – I have something approaching thirty examples of this brand name, and none of them has this diamond style of logo on the clip, or this exact type of nib marking.
Of course, Mentmore produced vast quantities of models and colours etc., and no doubt some are scarce and seen only rarely.
Might this one be available on your sales site – if so you might put my name on it:)
thanks for posting.