FX 19737: this is the rear end of the FX shroud compatible for the FX Impact compact, standard and super sniper (MKI, MKII, X, M3).
The following FX number are for the shrouds tubes only:
FX19131 shroud tube 28x157mm is the shroud of the FX Impact compact, 500mm barrel.
FX19692 shroud tube 28x270mm is the shroud of the FX Impact standard, 600mm barrel.
FX19142 shroud tube 28x447 is the shroud of the FX Impact super-sniper, 800mm barrel.
Check the thread on the shroud tube: thread M26:1,0 fine thread.
FX19588 compatible with the same Impact shrouds, but thick thread.