Strange filter thread on a Ring Flash
 

My wife is interested in Macro Photography, and I found a cheap ring
flash at a second-hand store recently.

It says "ACCESS Ring Flash" and "Made in Japan" with no further
information, or model numbers, or brand names, or anything.

I would like to be able to screw this to a standard 55mm filter mount.

Measuring the "filter ring" threads, it appears to be either a 54x0.7mm
thread, or a 2 1/8"-36 thread. There is no standard filter ring size
that is 54mm, and 55 and 52 (and 49, etc)mm filter rings all have a
pitch of 0.75mm, not 0.7mm.

Has anyone ever run into something like this?

If all else fails, I can have a step-down ring custom manufactured, but
it may not be cheap...

Spott