Anyone know this thread?

[Highburymark]

I'm trying to work out options for my recently acquired AP barcon Barlow (for use with Baader Herschel wedge and MkV binoviewer). I'll just be using the lens element on its own, which has a T2 (and 2") threads on the binoviewer end, but I'm not sure about the connection on the wedge/diagonal end (pictured). It's 38mm diameter - wondering if it's a unique Astro Physics or American format - can't find any info elsewhere. Can anyone shed light on this?

[Stub Mandrel]

38mm is a common filter camera thread size, perhaps its menat for an ND filter (I've just been googling ND filters).

[Ruud]

42 and 48 mm threads?

that's mentioned here: https://optcorp.com/products/astro-physics-2inch-advanced-convertible-barlow

EDIT

Oops, you mention a 38 mm diameter.