I have a pressure bleeder (Motive Products) that I got when I got my Mercedes. It screws on the reservoir. I thought it would be good to get an adapter for the Lotus so I ordered one. They offer a very extensive selections of adapters for a lot of cars including "Classic British" cars, and a couple of Girling adapters. Even one for Tilton master cylinders. It didn't fit. After many back and forth emails, taking measurements of diameter and thread pitch, etc. They had one in the right diameter but the thread pitch was wrong. They tried to be helpful but in the end, they weren't able to provide an adapter that fit my stock Girling TCS master cylinder.
I only go through all this to emphasize that I don't think there are any standards of reservoir caps and they seem to change them enough that attaching anything to the reservoir is a hit or miss proposition, even if it is from the same manufacturer.