Hi Susan.
Based on the literature available to me, the Rio Bravo was produced from 1973 to 1983. There is a guy on eBay that claims he bought a Rio Bravo in Italy in 1969 but there is no Rio Bravo in the 1970/71 Eko catalog so if it was being sold, it was only for the domestic market. I would say your guitar was built in the 1973-76 range.
Your guitar is a curious hybrid between a Rio Bravo 12 and a Ranchero 12. The metal trapeze tailpiece on your guitar was characteristic of the Ranchero 12. The Rio Bravo always appears with a Martin pin bridge style setup.
You can see the difference between the two guitars in this catalog:
It would be interesting to determine if the top on your guitar is built like the Rio Bravo which would have heavier bracing or the Ranchero which would have had lighter bracing since string tension is transferred down to the tailpiece.
You might contact Jack Marchal
jack@fetishguitars.com at the Fetish Guitar site to see if he has ever seen this combination of parts before.
Definitely curious