This thread is a lot bigger than the part it discusses. The hole is about the size of a 3/16" drill bit. 7/32" is too big to fit the hole.
Yes, the first CIS system had the plug. #39 in diagram for '73.5 PET.
911 110 934 00 Stopper for Adjusting screw. Same part # through 1980.
It seems in '81 when they went to O2 sensor and frequency valve they started using the tamper proof screw, which they recommend buying a new tamper proof screw.
There seems to be 2 places I ordered one from Classic DMC
https://store.classicdmc.com/catalogsearch/result/?q=102392A
I did not have a lot of faith in DeLorean Go. When checking shipping, on half the choices they said not to use please choose the most expensive option.