Putting this out there for posterity.

Yesterday I discovered that my 8-1/4" Red Dot Artar (s/n 8169xx) accepts 40mm filters. I'm not sure if the threading is exactly the same as the filter's 40mm x 0.5mm but filters from my Canon LTM kit screw in all the way and seem tight.

40mm is a pretty uncommon size but for some reason SRB Photographic in England make a 40-49 step-up ring (now on order). From there it's easy to get to wherever you'd like, in my case 52mm.