1981 I think, going by the RD. Catalog here:
http://www.miyatacatalogs.com/2007/1...alog-1981.html
I bought one with paint like that from a LBS, excellent mech...I paid $149 in 2005. It was a fine commute bike, recently retired, perhaps to be reborn as a ss/fg.
My sense is old 610s, 710s, 912s and 1000s have gotten more popular in the last 5 years, wayyy more popular. Any of those old cro-mo 7xx/9xx are good, "semi-pro" racey series.