Thanks Bryan,
I've been doing a bit of Web-trawling and found that Kowa Seiki Tool Co, Japan apparently manufactured all Honda tools in the 60s and 70s and cast either a circled RK (for Repair Kit) or 'KOWA' into the reverse side of the tool.
I haven't been able to find out what came first (RK or KOWA) or the switch date, but have asked the same question on the SOHC US site.