3.5x1mm o-ring (NBR 70) for various FX PCP air rifles.

This is the inner o-ring of the Dynamic FX20923C valve seat, as used in the FX DRS (MKI and MKII), Dynamic (MK1 and MK2) and Panthera (MKI, MKII and Hunter MKII).
The outer o-ring is the FX19108 o-ring, preferably replace them together during maintenance.

Tip: lubricate this o-ring with a small film of silicone grease to extend its life.

In doubt if this is the correct o-ring or how many you need? Check the schematic!