Our plates seem to be attached with rather large slot headed screws/ bolts.
I would doubt that they are just self tapping, I'm sure they would thread into something inside the rudder.
Our plates....and we're not that much removed from you # wise ....are just fine...I work them over with a wire brush on a drill every time we haul out...they haven't actually decayed at all...and the SSB performance is always improved after the wire brushing.
BUT the ONLY person who would know for sure how they are attached is OLIVIER BEAUTE....
OLIVIER, perhaps you can answer this question ?