There's another post that cam to the conclusion that 'I should coco' is probably not rhyming slang but it stopped there, anyone know where it comes from ?
It is rhyming slang for 'I should say so' and has been used as such since the mid-1930s, when it first appeared as 'I should cocoa' in a publication by J Curtis called 'Gilt Kid'.
Cheers, Darth, but it's probably owing to my having access to wider reference/research facilities rather than 'superior knowledge'. (I've had much longer to collect these than you have, I imagine!) I thank you, however; you're most kind.