Uh oh...I guess what I took as the dragon's eyes....weren't! I never have gotten Japanese dragons--the Chinese variety seems a little easier for me to unravel, but I won't test that in public.
I believe this coin belongs to the earliest type, that is, the type also dated 1870 (Meiji 3). I didn't bother measuring the diameter of the circle of dots, but instead keyed off the diameter of the coin itself. Krause-for what it's worth here--calls out a second variety (and presumably the third also) as being 30.5mm in diameter, whereas this coin measures slightly more than 31.5mm (and weighs, on my scale, 12.53 grams). I'm sure there are sub-varieties of this first [Krause] variety, but what they might be I have no idea. It would be fun to have better information.
And a question about the silver types of 1873 that succeeded these first types in the various denominations...my old (2006) Krause lists 10-, 20-, and 50-sen of fineness identical to the previous types, but of greater weight, and of sizes no longer metric-friendly (as were the previous types). This seems very strange to me--particularly the departure from easy metric measures. Any idea of what went on?
I go to study dragons....