I recently purchased a 150mm f/9 Graphic-Kowa with external (male) 46mm threads. I read a post here on APUG in which someone was using a 46mm step-up ring sandwiched between the front element and the shutter and keeping it tacked in place with a touch of glue to keep it from spinning. In another thread someone was using a 46mm step-down ring and screwing filters on backwards.
There's another solution... and apologies to those who already know about these. Female/female adapters are available in several sizes. So, in addition to a 46mm step-down ring one can add a F/F coupler so filters will screw on normally. I chose 58mm because it's large enough to eliminate vignetting yet small enough to not interfere with shutter adjustment.
Here's what I ordered...
I hope these work!!