Owner modification - uncertain as to why it would have been done. It really doesn't do anything except add weight.
Lower numbers on the pylons indicate an older boat, but determining the exact year from them is impossible. Hobie 16s were introduced in 1970 and had pylon serial numbers until the federally-mandated HINs started in 1973. Nobody remembers what pylon number Hobie Cat started with (a good guess is 1000). How many prototypes / pre-production models were made? You can see that this gets murky really fast.
One thing's for sure - they're either 1970, 1971 or 1972 models.
I'd use the filled in ones on the Tremelino - deck lip modifications like that are not class legal (not that you'd be racing a 45 year old boat) - but you never know.