I assuming from observation that the Series 2 chrome bowser cap is slightly smaller at the tip than the Series 3 as the S3 seems to jam on the edge.
Also the tube at the back of the fitting, is that for series 3 cars only and not required on S2? any tips or thoughts welcome.

 

In the beginning I tried fitting S3 caps to my S2 body (didn't know better at the time) but they didn't fit. Too big for the body indentation as you have discovered.

Vent/overflow tubes however, my car came with them and the Blue Book shows they belong there.
But What a PIA to get them off/on!!
(';')

 

Unless you need to remove the neck that goes into the tank, it's easy to tap the roll pins out at the hinge and get the lid off to fit a new cap rubber etc... Now getting the neck from the tank ...that can be a bear. Btw... my series two necks fit fine into new series 3 tanks, just for the record.

 

figured as much, these S3 caps chop down on the edge of the recess for the cap and stick.