R, yes, it's not standard that's for sure. I did find some pictures of Honda twins in classic racing in the US fitted with this brake and they implied (but weren't specific,) that the brakes (both front and rear,) were race kit items. I know what you mean about Grimeca but the beautifully cast on torque stay says that it was intended for specific fitment, I'd have expected a more generic stay mounting stud for an accessory item. Anyway, owner hasn't come back to me, I'd expect the castings to have an HM logo or Honda wing motif somewhere? Did they do this in the 60s I wonder?
REgards, Mark