yes that's true, lots of testing done and done well, but not with one wing tank empty and one full which changes things a lot.
Also largely done on short wing versions I think.
Early wing tank versions has gascolators and a dual tap near brake handle. Still see the odd one with "sharks fin" centre vent, gascolators were removed after a couple of problems, vented tank caps and header tank installed under pax seat. 3 port individual taps near wing roots, reckon they had 2 in one out, later the handles were removed. In J230 the header tank moved to rear and taps deleted altogether, Little clamps supplied in toolkit though.
There was some interesting videos on line on these spin tests.