Grae, Based on unreliable memory, I think from about 1985. 130hp engines sound less power than usual, but it can be confusing, the commonly fitted Volvo 165 could but did not necessarily mean 165hp, for example I think Mariner 31s had 165s of 130hp. Another engine used was Cummins 5.9L, and there were some Volvo 200hp. I`ve not heard of Riv making cored hulls, so that`s unlikely. I don`t think there was ever an aft cabin in the Riv built GBs, the ones they built were a partial redesign (which Marin has strongly criticized). They still bring big $ on the used market.
Andrew Moore, the broker located at Royal Motor Yacht Club on Pittwater could be a good man to ask. AndyG moors at RMYC. Hopefully someone with more direct knowledge will post.
Island Gypsy 36 Europa "Doriana"