Green’s Dictionary of Slang

a ticket inspector.

1889		Blackburn Times 16 Nov. 2/2: [headline] An Interview with a Female Railway Ticket Snapper.
1899		Irish Indep. 11 Aug. 6: Ticket Snapper Caught [and] charged with embezzling excess fares .
1903		Bournemouth Dly Echo 5 Aug. 4/1: Big Hauls Made by Railway Snappers [...] One ‘snapper’ [...] has collected as much as £10 in a little over a week.
1929		C.G. Gordon Crooks of the Und. 226: His ingenuity will be brought into play to outwit the ‘snapper’.
1938		F.D. Sharpe Sharpe of the Flying Squad 333: snapper (a) : Ticket inspector.
1947		Dly Mirror 19 Apr. 2/3: [headline] Snappers Don’t Miss Much [...] Most snappers [...] know the spivs are chewing the end of the ticket to erase the date. They refuse to accept the ticket.
1957		‘Nino Culotta’ They’re a Weird Mob (1958) 142: The snapper’s got yer ticket.