Air bags located within the front coil springs would be my guess. GM encouraged the chassis builders to install them when it was found they had overrated the front springs. For whatever reason, these were run for several years, and IMHO, are just a pretty much useless patch, leaving most owners with sagging front ends even when inflated to maximum capacity.
I ended up replacing the springs in a '93 HR 34' for about the same price as a replacement set of airbags (150. delivered). This lifted the front end about 1.5". My concern at the time was regarding a harsh ride with these MUCH stiffer springs - which turned out to be a non event. Wasn't harsh in the least.
350-6560SDM Heavy Duty Coil Springs