HM is Honda Motors, as you guessed. I believe it is a motorcycle wrench (~ 70's) , but the toolkit wrenches aren't all that well documented for the Honda motorcycles (There are also auto toolkit wrenches...)
8x12 is the size (millimeters), RK is supposedly for "Repair Kit" (what the wrench was for, the tool kit), The 'A' supposedly means something*, I don't know if 3f1 actully means anything ...
(Supposedly the wrench should be marked RK *or* A , not both LOL)