I went to the Science Museum in London last year, where I saw (amongst many other wonderful things!) the Apollo 10 CM, Charlie Brown. Looking at the heat shield, I saw two circular indentations with a tube poking through between them.
A hole in the heat shield would be a very bad idea during re-entry, so presumably a cover was ejected after the atmospheric braking.
I asked a member of the museum staff what it was at the time, they didn't know, and I haven't found any information online since. Does anyone know what it is, please?