Photopia was a British importer of camera accessories. Many items were brought in under their original names, but many were "house" brands, like this one. It appears to be a rebadging of the Japanese Gold Crest PR-95 and the Cidex.

Using this thing is fairly simple. There's a knob on the side with four positions: off, Check, High and Low. Since this is a CdS meter and takes a battery, you normally leave it in Off. Click over to check the battery, then go to High or Low to take a reading.

The knob is connected to a roller that has the markings behind the meter needle. In Low and High settings, there is a range of letters. You look at which letter the needle points to, then turn the pointer on the calculator dial to that letter, and that gives you the speed/aperture combinations.

This is the only meter I've seen that uses this particular method of marking the needle position. It's unique but no better than the old way of marking numbers across the face.