That's the way they came from the factory before the first brake adjustment was performed.
The welded plate was knocked out and after the job was done they replaced it with a rubber
plug.
Texas Swede
There was a TSB article released to the Service departments letting them know of the change in 66. Other car manufacturers did the same for years. Punched out likely over a 100 pairs in the 70's working at a service station while in high school.
In some cases (if the Ford measuring gauges could not be used) workers were instructed to punch out the metal plugs and cover the hole with the rubber plugs. It's believe that is why (sort of like the CAUTION FAN decals) there were so many boxes of the NOS rubber plugs found in the parts system for a long long time. It was introduced into production during 7/65 according to TSB #327
The metal knock-out is normally attached at the lower left bottom edge of the opening and once knocked out can often leave a small rough edge at that point
Your car would be about the 30th car I've documented plus the ton of others I've seen over the years before we started to record and take pictures
Believe there is a prior thread discussing the change
A note and line item as been added to all the 65 Running Changes stickys
An example - from my wife's car