As I mentioned in other posts and dovetails with this topic http://www.airgunnation.com/topic/tools-and-such/#post-218239,
If you have a PCP.....It will eventually produce a leak! So I have been trying to get a large stock of o'rings. Metric o'rings have far more sizes than the US standard o'ring counterparts. I think at this stage I have every o'ring for every gun I own. Still I come across one that may be a variant from overseas low power to US high power.
The following is just my theory.... Some airgun companies make it fuzzy what size the gun uses. Perhaps to protect and sell their own part number. This is fine but when a gun becomes a big seller, the information will get out. So it comes down to..... Do you want a customer with information, or make some extra money on parts. I don't blame them for either. I don't think any of these companies/dealers are getting rich fast. Another thought is, maybe they want the correct o'ring to be placed into their gun. We are dealing with a high pressure vessel! Some companies even go the route of supplying extra rings with the new gun. Well enough of my .02 cents.
The fourth picture down is a great o'ring spec reference. https://www.marcorubber.com/o-ring-size-chart-index.htm
A few standard will cross reference to metric. Since we don't have a good measuring device for MM o'rings besides a set of calipers, we can get a "sort of close" idea and look at the chart size. This generally gets enough information to get the correct size.
One of the most asked questions on this forum is "What size o'ring for this spot". The good thing is that more people that have a good reference sheet for their gun, they can share it to help out a fellow airgunner. I wish we had a database section on this forum. We have a lot of information on here it is just hard to find. But either way, we do have this forum which is great. It is just the nature of the beast. Airguns can be cantankerous but they sure are fun. It is also fun to bounce off questions and answers with other enthusiasts. It beats posting my vacation photos on Facebook that nobody else cares about.