Yes, 1/4"-20 jack nuts are what the factory used to mount the tripod mirrors. The originals have a rubber coating on them to seal out water (I think it helps to keep the outer flange of the jack nuts from digging into the paint too). Here's an example on Amazon -->
Amazon.com: Coated Jack Nut 1/4-20 With Raintite...
On Dad's 77 we used some stainless hex head bolts like this -->
http://www.ebay.com/itm/20-pcs-1-4-2...-/161024812280 With a stainless flat washer under them, they look similar to the original bolts (the hex head is a size larger though).
EDIT ... I just noticed the bolts in that ebay link are only 1/2" long ... I think the ones we used were more like 3/4" or 1".