I just checked the wrench included in the bike's tool kit (stock HD), which is the one I used to remove the fork top plug. It is exactly 36mm (1.417"), and if you used a 1 3/8" (1.375") socket my 36mm must've been a tad oversized, although I don't recall that being the case. Anyway, this wrench also fits my 36mm axle nut, FWIW. The 1 3/8" socket sold at Dennis Kirk says it'll fit any 35, 37, and 41mm fork cap--but I don't see how it would fit a 37 or 41mm plug. If the plug is exactly 1 3/8" it should only fit a 35mm plug.
Any of you doing this job, please measure the plug's socket size. I'm curious. Next time I remove the insert I'll to it, too.