This is what the OED says:
d. colloq. A customer, a client; (sometimes) spec. a prostitute's client.
1965 Sunday Times 24 Oct. (Colour Suppl.) 66/3 There is plenty of irrational judgement about..but like all free-market operators, the traders have to concentrate it on people—on each other and on the ‘punters’ (dealer buyers).
1968 D. Braithwaite Fairground Archit. iii. 60 Described by veteran showmen as ‘a good oncer’—that is, a ride the punters would normally go on once—and once only.
1969 Jeremy 1 iii. 22/2 Punter, client.
1970 Sunday Times 15 Mar. 60/5 I [sc. a prostitute] always make the punter wear a rubber.
1985 Times 28 Mar. 9/3 Eager punters would very quickly convert the deer into venison with a particularly delicious sauce of dried mushrooms.
1995 Guardian 6 July ii. 5/4 From the punters' point of view, the prostitutes are less likely to be using drugs.
2005 Arena May 39/2 With wide, muscular shoulders, V-shaped screens and deep-set lamps, it's exactly the sexy, sporting car Alfa needs to lure in the punters.
First reference is from 1970, so must have been used colloquially for a few years before then.
