The first model appeared in 1935. The model 11 which differed considerably appeared about a year later.

The first model was slightly larger than the later model. It differed in operation: in use the film speed is set on the calculator. A knob is turned until the required shutter speed is indicated by the needle. The f number is then pointed to by one of three arrows, these corresponded to three shutter speed scales over which the needle moves. The scales and arrows are coloured red, black and green. There is a further scale (A - G) for cine use, this is for cameras having a shutter setting of 1/32, the needle is aligned to one of these letters, the same letter on the calculator will then show the aperture to use.

With the Avo 11, the film speed is set, a pointer is lined up with the meter needle, the shutter speed / aperture pairs can the be read. The price was 55/-. Also available with continental f number values.

In use the film speed is set. A dial moves a pointer which is lined up with the meter needle. The shutter speed / aperture pairs can then be read.