The Field Rental-Only Gun (FROG) is a mechanical offshoot of the Mayhem, both from the company Paintguns International (PGI). Internally the markers are identical, however FROGs are fired using a mechanical trigger valve rather than an electronic solenoid valve.
The number of FROGs in the wild is unknown. The markers are very rare since they were presumably sold direct to playfields. Myself I bought a FROG pair in 2012 after buying them from classic marker curator Baccipaintball.com, who came into possession of them in broken and dilapidated condition. I fixed the markers up and took pictures along the way (below).
Here is a brief list of common parts:
· ACI bottomline regulator (used on PGI Mayhem, AirStar SuperNova)
· ACI grip frame used on AirStar SuperNovas and DarkStar Novas.
· Clippard firing valve: HV-HTV series
· Barrel threads: M22x1.5mm (Spyder)
· Feedneck threads: 13/16-32 (same as used by SP, DLX, KEE, PMI, ICD, etc)