MurrayMac,
I well remember when the poo hit the fan over this songs lyrics and Rolf Harris then bringing out a sanitised version, and all the ensueing controversy and publicity, but nowhere do I recall the hopping kangaroo / penis version you put forward. Back in the 60's we just weren't that subtle with lyrics and RH is extremely conservative and probably devoid of that degree of imagination.
I would think your version as more likely the creation of an expansive folkie 3 litres into a 5 litre cider flagon.

JG / FME