SU Carb for Sq4

[davewalters]

I'm looking for some guidance about the SU Carb on my "57 Sq4. When I rebuilt the carb the plastic insert for the top cap (used to put a drop of oil on the needle) mostly disintegrated. So much that I can't make out the pitch depth of the insert. It looks like it might have originally been bakelight, the "50s version of plastic. I put a rubber stopper in where the twist off piece belongs but would like to fix it correctly.

Anyone know where I can either get the insert or know the pitch so I can get a shop to make a new cap?
Thanx in advance.
Dave
dave@davewaltersphoto.com