Am putting a '72 Plymouth back together and am trying to figure out what you call this type of mount. It is located under the drivers fender where the battery tie down strap, where can I find something like this? The hold down bolt was rusted and seized up. Thank you for your help!
-Jim

It’s a of type weld nut, would require some careful welding and paint work to put it back to how it was. A 5/16” caged nut may fit the square hole as an alternative if needed.

Its called a pierce nut
http://amkproducts.com/bulk2.asp?code=7135&title=Pierce+Nuts

when using washer and nut under the fender for 20 years