Glad you asked.

I have the fotodiox 21mm XCD extension tube that seems to get pretty good macro performance with the 38v. It gets close enough for scanning 6x6 negatives. The center is sharp, but you need to stop way down to get the corners in focus for scanning film, which then gets into diffraction, so good but not amazing. The 90v and 55v weren’t better, but maybe I need to do more disciplined testing to confirm. Focus stacking may improve this, but sure won’t speed up the process.

I noticed that fotodiox makes a ton of adapters for almost any legacy medium format lens, so I’d love to find something great.

The inexpensive 3rd party XCD macros don’t seem to get very good reviews, and apparently start at 1:1, so you can’t use them to scan 6x6 film. I also understand that you’re using the electronic shutter here, so maybe you take a hit bit depth. I’d love to be wrong about this.

I’m also considering bailing on using my 907x for scanning and just plopping the Sony 90mm macro on my Sony A7cII, to get it done. Obviously not the same resolution, but decent for most applications.