Alas, no. I tried some 18 mm spark plugs and they don’t fit.
On mine, the thread is 10 tpi and the diameter is either 3/4″ or 23/32″ (or something else entirely).
An internet search suggests that 10 tpi bolts are either BSW or UNC.
One would think that sidecar fixing bolts would all be the same in the 1930’s.
All very interesting stuff.