This was the last model of dedicated Leicameter that Weston made, and the only one in the 'Master' family. Its mode of operation is essentially the same as the other three earlier models. The two main differences are that, instead of using Scheiner film speed ratings (no longer in general use by 1941), Weston's own speed ratings are used.

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For a reference film speed of 24 Weston and f6.3 (on the B scale: Baffle closed), the shutter speed is read directly off the meter. If,as is probable, a different film speed rating is needed, this is set on the inner red disc of the dial. The shutter speed indicated on the meter is then aligned with the outer dial pointer. Any desired combination of aperture and shutter speed can then be read off.

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It is not clear how popular or convenient this inverted method of using a light meter was. This model of Leicameter itself seems to be very rare and far fewer examples seem to be around than the earlier and relatively numerous Model 650. The Weston Master Universal on which this model is based was massively popular and established Weston as the foremost producer of precision light meters in the world. Its Universal calculator dial and manner of use became established across most meter manufacturers and remained for the next 60 odd years. 

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By the mid '50's the screw mount Leicas with their non-linear shutter speed scale had been replaced with the 'M' series which followed the conventional pattern for shutter speeds. There was no need on later Weston meters to design a model specifically for these popular cameras.