See the attached pdf file for a section of the manual with o-ring sizes noted in mm.
See this article for DH manuals,
DH Type 5000 Pressure Balances (Piston Gauges) - Manuals
Here's a photo of the tool that is used to remove the mounting post o-rings.
The part number for the 5303 variable volume seal kit is 3075460 (old DHI PN was 100192) and the description is 100192,SEAL ASSEMBLY, OIL, VV, DSG. It does not come with instructions (instructions do not exist) so be careful during installing that parts are installed in the correct orientation. There is typically a 4 to 6 week lead time as we don't normally stock this kit.
Below is a snippet from the attached document that shows more details and information about mounting post and priming pump o-rings.