I got confused and for some reason assumed that the Solex carbs on the Ace Bristol engine were 32 PBIC when they are in fact 32 PBI-6, which is actually just what's already mentioned in the AC owner manual (I know, RTFM and all that...).
The only difference I could spot between the two is the separate throttle body housing on the PBI-6 mating to the actual body of the carb with a 4 bolt pattern.
There does not seem to be picture on the Internet of PBI-6 carbs, but this illustration of the 32 BI shows a similar construction, and suggests that the vacuum port should be plugged with a screw plug labeled number 16.
Looking at the PBI-5 as used on some Jaguars, the vacuum port seems to be left open :
So what is on your engines, gentlemen?
Are your holes plugged or not? (pun unintended)