Pat, Thanks for your reply but, I actually did a reverse image search of the diagrams I used and found the original post by Bell#3156 here. ( 3/8s tube it is! )
The post header is "Swirl tank/ header tank"
I was using the wrong search words. 🤔
Sorry for lack of info. Just a little background, I purchased #2636 (too many years ago😔) barn find in non running condition so I have no knowledge of coolant system performance but it was a mess.
High desire is to maintain as original appearance as practical but do many of the TSBs and upgrades to make it less quirky and more reliable.
Jerrysr