A Mercury II, made by the Universal Camera Company. The round housing on top conceals a rotary shutter. There are two metal discs in there with something like pie wedges cut out of them. Adjusting the shutter speed moves the discs in relation to one another so that the missing "slice of pie" gets wider or narrower. Tripping the shutter rotates both discs. This one has been releathered, cleaned, lubed, and the utter hell has been polished out of it. It probably works better now than it did when it was new.