I knew about Jubilee variations generally, but not the trick on spotting tender capacities at distance. Though midland mainline loco's having 3500's because the 4000's were needed elsewhere makes sense.
(Here's hoping the free hand of capitalism sorts out Bachmanns short/long firebox self-fufilling prophecy; speaking as someone who would love a modern Leander or one of the 10 Derby-Built examples, regardless of East-Midlands applicability.)