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.
Below is a snippet from the attached document that shows more details and information about the mounting post and priming pump o-rings. We do not have Fluke part numbers for these o-rings. It is recommended to source them locally or on the internet with the provided information.