I’m confused. Those oval shaped covers that attach to the front and rear floor pans with two screws are called by most vendors “drain plates” or “drain covers” but considering that they are covered with sealant and screwed down to the floor, how do they drain anything? What are they really there for?
I remember reading something somewhere that stated the floor drain holes were put there by the designers to allow the priming solution to drain out of the body when the shell of the body was dipped and submerged in the primer solution. I’m pretty sure these cars weren’t dipped in primer but were sprayed with primer instead at the factory, therefore the floor drain holes might have been carried over from previous methods in the past.
To drain after the rust proof dunk.
LOL!! Judging from some of the floor pans I’ve seen over the years that “rust proof dunk” wasn’t very successful! They must have drained too much of said rust proofer out of them!
I wondered the same thing for the longest time! I was like “drain plug”?? Why would you want to plug up the drain? No wonder they rusted out with all the drains plugged up!
Only fairly recently did I read about the whole “dunking” process of priming / treating the bare bodies by the factory. Same with all the wierd ribbing / grooves on the floor pans and other panels - their purpose is to channel the dunking fluids to the drain holes. IIRC, I read about it in a recent issue of Hemmings Muscle Machines - they mentioned it in an article about replicating the factory paint process for acurracy in the paint details on an Olds 442 convertible (IIRC). When I read that it was a very “Ah HA!” moment for me 
Ok, since we’re talking drain plugs, when did they start putting date codes on them? I’m taking apart an A17 built 69 and the plugs have a date of 12 5 on them. So I went to look at the dates on the plugs on the H26 built 69 and no dates. Just curious, would assume about Jan when new mandates were implemented.
Oh, that’s easy. You remove them when you want to let the salt water out of the floorpan after Hurricane Isabel put your car under water.
Wait.
That was just my car.
Nevermind.
Plugs were an engineering left over IMHO from the typical Ford design of the floor panels (for those cars - more expensive models- that were dipped)
Cougars uni bodies were not dipped like T Birds or Lincolns) just a spray top and bottom (bottom from jets mounted below track) with plenty of areas missed. 