From my memory, and we can wait for Spraggy to give us a better explanation. The old system of currency in the UK (pre sterling like 1970) was with shilling and pence. Basically 12 pence per shilling. The D is basically a cent sign, so 2/6D is a 2 and a half. But the 11/- is like 11.00

http://en.wikipedia.org/wiki/Shilling