The fixed 1810 date is still useful, Mick. Cheers.

Further deliberations. If we can't place 'rally-pin' and 'gally-pin' as actual words then it would seem logical to say that perhaps 'gally-pin' is a misheard 'gallopin' and that 'rallopin' is simply a nonsense rhyming word in the same vein as 'super-duper'

Alternatively another longshot 'rally-pin' could be derived from 'reloping' which would go well with 'gallopin' as both being terms associated with hunting. I have no idea what 'relope' means as I only know of its use in the Copper Family hunting song 'Sportsmen Arouse'. I'd guess it means hold back a bit, or reign in. the word isn't in my Chambers or any of my slang dictionaries.

The word 'rollocking' which features in the chanteys may turn up something in its derivation.