Probably too late a reply for you to use but.....
I have a 1967 Islander 33 flushdeck and the boom you describe is what I have as well. It roller furls from a crank on the gooseneck and the mainsheet is on the rear of the cockpit right below the end of the very long boom. I've installed reefing kringles and lazy jacks as I found the roller furl boom to be awkward to use.
As far as I know, the roller boom with aft sheeting was stock. I still use the original blocks for the main and jib sheets as well. Love the boat! Sorry, but 'Summerspiel' didn't come with a manual for me either.