I should have made a *note about this before: the accordian stem pictured is the stem I'm trying to replace, as it doesn't have the wedge and tightening mechanisms. The stem falls out.
I think I understand this though.
*Out of these four stems, the BMX one is the oddball. It is .833" (21.15mm).
*The others (the MTB one I'm trying to replace and two road stems) are the same standard 7/8" (22.2 mm).
*Therefore, I'll have to replace the MTB stem with one of the road ones. Also good news: I think the handlebars are all the same size, so it should be a relatively straight foreward swap.
Sheldon, thanks for the links to your tables. They probably helped me understand better than any amount of words.