I bought this in 1989, and never used it, it is in like new condition. Quoting from the August 88 AP price list:
2" Flat Field Photographic Telecompressor with "T"ring for 35mm cameras ... $175
Mine has an Olympus OM-1 T ring, but you can get a T ring for most cameras. I think (not very sure) that this dropped my 155 f/9 AP down to about f/6.5, which makes for shorter exposures. It needs to be used in a 2" focuser, or a larger one with a 2" adapter.
The current AP device has these words; "The distance from the edge of the 2" adapter to the film plane should be about 65mm to achieve 0.75x. You can decrease the distance for less compression or increase the distance for more compression" I am GUESSING that statement is also true for this one, but I am not sure.
I will sell it for $100 and I'll pay the postage. I think this is cheap, if you can show me evidence to the contrary, I'll lower the price. The current AP one, which is a 2.7" version, is $250.