This is the O-ring Gasket Part AC0141 that seals the cap to the main body. Note: these are not preformed as the Senco OEM gaskets are, but regular O-rings the correct size and thickness, just fit them in the groove cut into the cap. Use the link below to add to your order other FramePro kits I carry.
https://www.rebuild-kits.com/category/senco/senco-parts/framepro-parts/
For the Models below
FramePro 600 Framing Nailer
FramePro 650 Framing Nailer
FramePro 600E Framing Nailer
600 (FRH) Full Round-Head Nailer
650 (FRH) Full Round-Head Nailer
FramePro 700E-XL Framing Nailer
FramePro 750E-XL Framing Nailer
FramePro 700-XL (FRH) Nailer
FramePro 750-XL (FRH) Nailer
FramePro 701XP Full Round-Head Nailer
FramePro 751XP Full Round-Head Nailer
FramePro 702XP Full Round-Head Nailer
FramePro 752XP Full Round-Head Nailer
SCN60XP Coil Nailer
SCN65XP Coil Nailer