Yes, when I bought my 228, one of those backs had jumped off as the previous owner was headed for Key West. He didn't miss it till he got down there. They spent a week there and, on the way back home they stopped in Largo for some lunch. A guy walked in the restaurant carrying the seat back and said he was following him when it bounced out. He recognized the boat and rig and gave it back. A miracle. It was beat up a bit so after I bought the boat, I had it recovered. I never trailered with them in place.
You are right about the snaps on top of the transom and the opposing half is under the curved portion of the seat. I copied a later model and had tabs sewed on the bottom with snaps. I had forgotten that till you mentioned the top snaps.