There are two different things Smart Parts did. First they make a thing called a "Magic Box" This was a black box that was attached to the side of the air chamber on a Mag.
*Photo credit to DamoNAN on MCB.
Then they came out with the "SmartMag" This was a totally new valve that was actually manufactured by AGD and sold with the Smart Parts name on it. It was just an enlarged air chamber. It didn't make it any better, and in fact a lot of people said it affected efficiency. They are not very rare and would sell for a bit over a normal classic valve price.