Same type of boom/cap construction on my E27. My cap has three screws that hold it. Is your throughbolt actually coming out? Mine has some amount of looseness that allows the tangs to rotate freely. If your screws were that corroded, it might take a little persuasion to remove the cap. If I remember right, my bolt has a washer and nut on the inside that is cotter pinned in place.
Do you have the original topping lift consisting of a short wire and snap hook attached to the backstay that clips onto the boom? For jiffy reefing, I found that system to be dangerous in certain conditions. When the wave action starts the boat to rocking, one arm tries to corral and center and lift the boom at the same time while the right tries to get the snap hook onto the tang. I always felt one more arm to hang on would be nice - especially since I had to stand on the cockpit divider or seat to reach everything. Lucky I had a pedestal guard to wrap my leg around. My topping lift now goes to the top of the mast - hope yours does too.
