Eyes Behind the Wall (1977)
L'occhio Dietro la Parete
https://www.imdb.com/title/tt0076485/
In this Italian film, Ivano (Fernado Rey) is a wheelchair-bound man who has an apartment filled with audio-visual equipment. This allows him to spy on tenant Arturo (John Phillip Law) and his various sexual conquests. He forces his wife Olga (Olga Bisera) to watch the action. Eventually, he convinces Olga to seduce Arturo.
The highlight attached here is when Arturo peels down the panties from blonde Olga revealing her full light-brown bush. Bisera was born in 1944, so she is about 33 at the time.
The scene with just the highlights and a slo-mo version is included.