There does not have to be a postal purpose for the value, since that issue was also intended for and used for revenue/fiscal purposes. I believe the 12/6 was intended for passport fees, since most are found with that type of cancel:
http://www.bermudastamps.co.uk/issu...enue-stamps/One could argue that there really is no postal purpose for the pound value. Like in the UK and its empire then and earlier, that one was probably intended for and used mostly for revenue purposes, or to pay both postage (small part) and revenue fees together.