The lead image here comes from the 1976 Mighty Marvel Bicentennial Calendar...
Captain America and the Declaration of Independence! Featured on the month of July for this calendar.
The Mighty Marvel Calendar for 1975 also featured Captain America and the Falcon. This is the original art to that one, wish I had a full color scan from the calendar.
And last but not least, here is the original art to the Aurora model kit for Captain America. You may remember this one--it came with a small comic book inside, illustrated by Romita. Nuff Said!