The three dimensions
All three in millimetres. If a dimension falls between two of the sizes on our parameter table, send us the one you measured rather than the nearest one on the list. We manufacture to order, so the number you read off the cartridge is the useful one.
The photograph
One shot of the top of the cartridge, taken from above, with the whole flange in frame. The mounting is what identifies the type: a flange with lugs, a bayonet fitting, a threaded neck and a polyurethane cap all look different at a glance, and none of them are easy to describe in words.
If the cartridge still has a nameplate, photograph that as well. It saves a step. It does not replace the measurements.
Then what
The specification builder asks for the same three dimensions and takes the photograph as an attachment. A copy of what you send goes to your inbox, so the specification exists on your side as well as ours.
If you are not sure which mounting you are looking at, say so. Not knowing is a valid answer in the builder and it does not stop the enquiry.