MG T-ABC

Disaster yesterday broke the piston in one of the carbs on my TF. Currently none available through the normal sources ( Burlen,Octagen,Moss etc.) Are they interchangable for example from a HS4? I also note that whilst these may be available -used, they have additional holes in the piston base. Can anyone explain their purpose? If they fit my car can I just seal those holes?

Thankfully local MG expert to my rescue with a pair of matching used. Having said that an interesting Youtube by John Twist on the subject. https://www.youtube.com/watch?v=GHNlT5yHDxk