It's a Heinrich hand-made 'big' tank. I found a bit of history here
https://turtlegarage.com/motorcycles/hoske-heinrich-pioneer-artisans-of-motorcycle-accessories/ . It was on the bike when I got it 2 years ago (from the original owner of 43 years) and is a genuine Heinrich - I think I have the receipt - and Heinrich is stamped in the tank at the back.
But last Saturday at the VMCC Autojumble I bumped into Ian Clarke (from Verwood) who had met the guy. Apparently he set up shop across from the main dealer in Stuttgart (or was it Mannheim) making the tanks and retired way back (late 80's or early 90's, I wish I had listened more) and offered Bob Perocha the business which BP turned down. The business was bought up by another German/Dutch guy who has not made a tank since then. Ian knows all this. And probably BP does, too!
So at the Airheads Association (Facebook) 2023 meet-up near Sammy Millers and Salisbury I'm going to invite Ian to come and talk about his 40 bikes and relate the Heinrich story again, if you like?