I can’t help with information about the history of that kayak, but I can share a few observations from a trip in North Queensland back in the late 1990’s when a guy from the Sunshine Coast turned up with a Rosco Expedition. I didn’t closely examine his boat but recall it looked solidly constructed, although maybe a bit on the heavy side for composite. The Rosco noticeably lagged behind as we left sheltered waters and attracted several comments about how low it was sitting (it was loaded up with more gear than I usually take on a 3 day trip). A noticeable lack of bow lift also ensured the paddler got a wet ride … probably not much of an issue in tropical NQ waters though. However as we moved out into more exposed (and quite lumpy) seas the handling of the Rosco actually seemed to improve, as evidenced by the fact that it wasn’t long before it was up near the front of the group. Much to the surprise of us all, the Rosco was able to make far greater headway in heavy sea conditions than many of the other (better credentialed) kayaks in our party. So it looks like you may have scored yourself a sweet little boat for getting out of trouble when things turn nasty.
Rosco Canoes closed down back in January so, unless you can somehow contact the former owner (Ross Cook) directly, I’m not sure how you can get more information about the history of your boat. Perhaps you could try Kev from Acacia Ridge Canoe Sales … I believe he was working with Roscoes back around the time your boat would have been made.