That would be Whitworth, I reckon? Years (many, many years...) ago when all nuts and bolts in the UK were imperial, there were lots of competing 'standards', but mostly "BS" (BSW, British Standard Whitworth, BSF, British Standard Fine, BSC, "C" for cycle used on motorbikes...) - or BA (British Association)... plenty of others!
The "AF" sizes, "across the flats", that was really an American size (what I believe the Americans call, er... "English"!), although it sort of became a competing standard in the UK later on (another one :rolleyes
. So yep, it was just as bad for me years ago, lying under my British Leyland car I too would be fumbling for the right size among my BS spanners only to find it was a 1/2" AF I needed 
I still have a huge chest of imperial spanners in all sorts of wierd sizes. That is the trouble with standards, as soon as someone brings out a competing standard, they're not standards anymore... Thankfully, post metrification, they are nearly all obsolete now, except the AF, because the Americans keep that going I suppose.
I remember the first Japanese car I changed the clutch on. A Toyota in about 1979... I did the whole job with about 2 metric spanners. And you always know you'll have the right sizes.
Give me metric any day!
Interesting question, though, what is the thread specification of a standard tripod bush?
