See this image for mounting post Polypak seal and o-ring. A few of both would have been included with the setup / accessory kit that is included with the equipment when new.
See attached pdf file for information on extended mounting post seals for higher pressures.
Part Number: 3865163
Description: 54-703-008,O-RING, BLACK, VITON 3/16 X 1/16 X SEC
Part Number: 3910811
Description: 72-33,SEAL PARKER 12500250V-250B Z4653, POLYPAK SEAL
Contact Fluke Calibration Orders Team for pricing and/or to order parts. firstname.lastname@example.org or phone +1-877-355-3225