Ok I think I understand, they’re studs with different threads on each side.
That I’d have to go googling for, I can’t even understand why someone would engineer it that way instead of just tapping the riser 10x1.25 and running a bolt from the start. Bizarre.
My suggestion would be drill it at 1/2, and use a longer bolt sourced from pro-bolt’s selection of Titanium bolts. They’re stronger than Stainless Hardware (but not stronger than good Steve fasteners like grade 8). If you decide to go back the slightly larger hole won’t really make any significant difference as 10mm vs 1/2” is really not that different.