TRUE---we use 5052O tubing versus 3003O that is Vans supplied. BUT, the assemblies are made to fit the locations. NO, they are not builder made so that is a gray area. BUT---if they are installed on the bulkhead for inspection, I'll bet the DAR might not even give them a second glance---or even know they are 5052O.
Yes----the intent of the regulation is that all the parts be supplied by Vans and assemblied to their plans. In that respect, we 'could' make them for clients from 3003O and meet the intent of the regulation.
As for the F12127D tube, yep we build them, but its a long tube and takes some 'doing' to install, and some builder fabrication.
Tom