Chain Plate Sealing
On my E29, My chain plated have a top piece that slides over the actual chain plate and cover the hole in the deck. About ever 10 years or so, I have to remove the two screws and slide that plate up, clean and re-bed the hole with a flexible caulk. Silicon works well.
As to the Chain Plate inspection.... I remove the wood panel inside the boat which allows me to see the glassed in chain plate. I tap with my inspection hammer and listen. I look for any changes in color or water infiltration.
FYI... on My 1971 E29 when the Port side chain plate leaks that water ends up under the Galley woodwork. so, if you have dry rot on the bottom of the galley,,, I would check for deck leaks around the chain plates... If one is leaking,, I redo them all.
Rick+