This is theSenco OEM Gasket Part AC0141 that seals the cap to the main body. 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