A 1965 HW35. So you would need to take it apart and have a look at the piston as it might be an O ring conversion but depends at what point the previous owner had it tuned. I haven't a record of serial numbers and guns tuned but full custom rifles serial numbers will give an indication of the period tuned
Venom Industries, Venom Arms, Venomac, Webley Venom.
Mach 1.5