Interesting, it's quite different to my early 21 (no 18). I think you're right that the block was originally shackled to the eyelet, but that has been replaced by the silver rope. And yes, the hoist rope is anchored at the deck.
Dropping the keel out isn't easy, especially if you're on a trailer. I managed it last year. I'd recovered the boat so it wasn't central on the trailer, and far enough back for the keel to miss the trailer's crossbeam. So, after removing the trailer axle, I jacked the whole lot up about 1m with the trailer on breeze blocks, then dug a hole underneath the keel. After removing the nylon keel slides and rollers (through the inspection hatches), the keel did drop out. Getting it back in wasn't easy either - it's pretty heavy. So I won't do that again in a hurry!
An easier way would be to crane the boat up, leaving the keel behind!