Powerglide transmission ID

[JHS]

I have an old 2 spd Powerglide transmission, cast iron, weighs a ton (even my girlfriend can't lift it) that I am told only came with the 55-57 Chevys. It has a cast number below the fluid tube and some sort of medallion cast into the housing by the engine flange above the tube. Are these units worth anything? I am told they are becoming popular in the bracket racing circuit but seems as the weight would be a hinderance. Someone wants to buy it but no clue as to it's value. Thanks.