BAFSV stands for 'British Armed Forces Special Voucher.' Beginning with the Occupation of Germany after World War II
the British Army
retained standing troops on the Continent. These troops, initially, were supplied completely by the British Army supply system. However, soldiers require paying, and soldiers with money in their pocket are usually a recipe for 'incidents' involving local taverns, merchants, black market
s, what have you.
In an attempt to mitigate these incidents, and to control the amount of currency flowing into the war-smashed economies of Western Europe (originally), the British Army issued part of the pay of soldiers serving in the BAOR (British Army Of the Rhine - the occupation force) in these 'Special Vouchers.' Essentially, they were private fiat money that could only be spent in the British Military canteen system.
These continued in use through at least the mid-1970s, according to various memoirs around the Internet. They're collectible now.
Addendum: Wertperch says "And actually, it wasn't just the canteen system. The NAAFI (kind of like the PX) also took 'em - so you could buy your beer and fags and food too."