Hmmm, I guess it very well could be a .833 BMX stem considering Mongoose's BMX heritage. Does the bike have a model name? Original paint?

Seems like the really old Mongeese (early 80s action) would be the ones most likely to have a BMX stem. A lot of them are pretty recognizeable by their virtue of being all chrome. I know by 1988 most, if not all Mongoose MTBs had standard 1" stems.