There are third party versions of the V-mount adapter too. RAF Camera sells them (on eBay and via his website). There are also Chinese versions on eBay, which work fine.
Look for "schneider v38" on eBay to find the RAF versions, and "schneider v mount" to find some Chinese ones. One advantage of the Chinese models is they only move the lens forward 5mm on the flange, whereas the Schneider version is 6mm. For longer lenses that's irrelevant, but for the APO-Componon 60/4, you may need that mm (see below).
I recommend the Schneider-Kreutznach Apo-Componon Makro-Iris 90mm f/4.5 without reservation. The image circle is large and the image quality is superb. Just be aware that it comes in multiple versions. Mine was labelled "Type -0024", which I've learned is the same as the Type -0025. The various models are calibrated for specific magnification ratios in industrial applications. My Type -0024 is superb as a taking lens.
The Schneider-Kreutznach Apo-Componon Makro-Iris 60mm f/4 is a very nice lens. I don't have it anymore but I liked it a lot. The problem is the image circle is a bit tiny. On a 33mm x 44mm sensor you shouldn't expect more than 8mm of shift. That was plenty for normal applications, but it's not enough if you want to flat stitch. The lens also has strong light falloff at wide apertures. Where I used it (f/8 to f/16), it was not an issue. The other problem with this lens is the flange distance is short so mounting it can be tricky. Ideally a recessed board is needed so that there's enough room between the standards for movements.
The nomenclature around threads can be confusing! As I understand it, Leica popularized the 39mm x 1/26th inch thread. It's common on enlarger lenses for small formats. I've seen it referred to as "Leica thread", "LTM" (for Leica Thread Mount), and just M39. You have to make sure that anything that is labelled just M39 is actually 1/26th inch thread pitch, or it won't work.
