Because Cornelius and Galen were both portrayed by the same actor, Roddy McDowall, Mego chose to use essentially the same head for both action figures.
The costumes and shoes are also virtually identical. It is speculated that Galen received some brighter colors of vinyl (from burgundy to a near purple tone) while Cornelius tended to get a simple brown. There are even different shades of thread in the stitching of the tops.
There is no absolute way to differentiate the figures, however, since both sets of Planet of the Apes figures were released on the Mego "Type 1" body - that is, the body which used metal joints, most prominent in the wrists -- AND on "Type 2" (plastic jointed) bodies. What does all this mean? Simply, a male Chimp on a Type 2 body is probably Galen, while a male Chimp on a Type 1 body is probably Cornelius, but it is impossible to be sure unless the figures are carded.
A complete figure (Cornelius OR Galen) has green tights, a green top with vinyl details, and moccasins (of either brown or burgundy tones). There is a very rare but confirmed variation of Cornelius in an outfit made of brown fabric in place of green, but with the same vinyl details. This outfit has been found on Cornelius only, not yet on Galen.
Above are some screen caps of the Apes TV Commercials featuring some early Mego Cornelius and Zira prototypes. Note the early head sculpts and uniforms and that the astronaut is obviously just an Action Jackson.(all photo cred for the commercial photos go to Mike T1K)
Tom has provided the Museum with a guide to Planet of the Apes card variations, great eye Tom.
The variant of Cornelius in a brown outfit courtesy Joe DeRouen. It is a striking contrast to the green.
Carded Cornelius courtesy of Troy has green ears which puts to rest the whole Galen/Corny differences theory.Both first issue and second issue cards can contain the mini catalogs.
Boxed Mexican Cornelius like all apes is sold without pants! (courtesy of Luis Felipe).
Cipsa (Mexico) Mego Cornelius courtesy of Mike J
Boxed Galen courtesy of Mike J
First Version Mego carded Galen courtesy of Mike J
Third Version Mego carded Galen courtesy of Mike J
Palitoy Carded Galen from Mike J, Cornelius was released on a Bradgate card
Japanese Bullmark Cornelius courtesy of Mike J