Ta for replies, hadnt seen that vid.
Those spigots/ports? No.1 is cast blank. So only 3 ports.
In the roof of the float bowl no.3 has a ball bearing , the others are open. Perhaps that one (no.3) is the vac tap?. (As gs7_11 suggested). If the other 2 are vents they need to be unblanked?
You were right, the plastic floats from bs34 carbs hardly fit (restricted travel), but good to see vid shows direct plastic replacements available.