How do I know which VOLA conversion I need when I see this?

There are actually three conversions that look similar to the one in the photo. But which one is it?

There are 3 possibilities:

1st possibility the 5400, the most important thing you could measure is the "5205" that clearly sticks out in the photo. 39mm is with this type of converter:

Option 2 is the 1400 from the 2000 series. If we also use the threaded section as a reference, you would measure 32mm there:

The third possibility is a much older version from before 12/1997 and would be the 800 diverter. It's also 32mm wide, but has a grip with an Allen key under the pin. Later versions have the Allen key on the underside, which is a clear difference. The 800 type also has a groove in the rubber of the diverter, while the 1400 has a hole, but for that, it would have to be removed first.