Found my dads original 1971 duster!

Bennoel 10 said
Looks to be a small leak as you state. Possibly just add a little heat and the solder might just pool up and seal. To answer your question I believe it's the same as copper pipe solder. That top rad tank is brass as far as I know. Hopefully some of the membership can either confirm or discredit my answer.

Thanks, Noel. Yep, just a pinhole. I just tried heating it up with a soldering iron. Not sure how well it got down into the hole. Here’s a better pic of the hole. If it doesn’t hold, il try cleaning the areas up and adding flux, then a re solder. Can’t hurt