Here we have two sets of factory 20v extractors, : silvertop on the left and blacktop on the right


If you look long enough or click the pic you might just be able to tell, blacktop has one stud still bolted in for reference.
Primary pipe measurement of silvertop extractors


Pretty safe to say it's 35mm
Measuring Blacktop primary pipe


Safe to say it's 38mm
Measuring Blacktop extractors port size


resulting in 34mm diameter at the flange
Measuring Silvertop Extractors port sizes


resulting in 32mm approximately, was carboned up.
Also measured the S/T head and had 29mm but again was probably due to a build-up of carbon.
Measuring the downpipe where the 2-1 section bolts up, Silvertop


And Blacktop


Both the flange ends are 40mm which could well indicate that the Blacktop extractors can bolt up to the silvertop head and appear to bolt up to the silvertop 2-1 section. (Previously tested this but didn't have any camera to test at that early stage. And you've got a great quick and easy enhancement for your Silvertop 20v