That is an engine ID tag, that identifies the level design, build date, etc. Usually, there's a bolt that holds it in place near the front of the engine, typically on the intake manifold (at least for 65-68's).
Rdm1952, most of those tags that were on any of the cars I owned were held in place by the bolt that fastened the coil bracket in place. I would sure keep that in a safe place until you reinstall it on your engine. Marti Auto Works would like $45.00 of your hard dollars for a repo replacement tag. [emoji594]
rdm1952, I'm out of state away from my vehicles, but I do have a couple of file photos I can share with you. Your fortunate to still have the tag with your car. As I posted earlier, they seemed to be a favorite thing for mechanics to throw away!