interesting , i didnt think of that. though the standard atb has a bi-plain fork , at least in the year i linked to.

but, this one has those slight crescent cut outs at the top of the seat tube, where as i dont see those on the mongoose. maybe it's not one at all. or a different year given the seat collar similarity.

edit. the standard 86 atb did have a unicrown fork. http://www.vintagemongoose.com/pdfs/mongoose_cat86.pdf
and cantis