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
Note: This seal is no longer available for purchase from Fluke Calibration
Parker Seal OEM part number is 4653-125-00250-V-250B
4653 is the shell material
125 is the gland depth or seal nominal radial cross-section, 125 is 1/8 inch
00250 is the seal nominal inside diameter, 00250 is 1/4 inch
V is for the o-ring (o-spring energizer) material
250 is the seal noniman axial width, 250 is 1/4 inch
B is the type of seal
Contact Fluke Calibration Orders Team for pricing and/or to order parts. firstname.lastname@example.org or phone +1-877-355-3225