I finally got an ABU Ambassadeur 5000C made in 1981, which may be one of the last 5000C's made in so-called old style, excluding later replicas like 1984 models. The foot number (811204) shows that it was made in December 1981.
I find three major differences from previous models (1976-1978 ones that I have in my collection, and presumably from 1979-1980 models as well).
(1) The drag wheel has 4 arms (4-point star).
(2) The color of gear plate (brake plate) is silver, not black.
(3) The color of screws are not shinny chrome, but dull-looking brass.
These characteristics look almost the same as another example of 1981 model that Mr. Fred Ribb kindly shows here (#811104). Thanks, Fred!