I see that all 65-66 and 67-68 w/AC heater cores use end caps (cardboard?) (attached to the heater cores, I think to minimize vibration. What I don't know, is whether or not the 67-68 w/o AC also used these end caps.
Provided better sealing between the hot and cold parts in an A/C box at the bypass. Not used in a non-AC box, at least that I've ever seen... just the two seals front and back between the core and the case.