I've acquired a convertible Gundlach T-R Anastigmat Series II---focal lengths 12"/21"/28", f/6.8, labelled for 8x10. This seems to be the No. 5 lens in the series, per a Gundlach catalog at http://www.cameraeccentric.com/html/info/korona_2.html.

There are filter threads, the diameter seems right for a 55mm filter, but either the diameter is just *slightly* wrong or the thread pitch is different. I can't find anything on the lens that would indicate the diameter. It's certainly bigger than 52mm, and I don't think I've ever heard of a 53mm or 54mm size---or were there once filters in inch sizes, and this should take a 2 1/4" filter (which is what the catalog gives as the diameter of the elements)? It's too early for series filters, right? Or are the threads not for filters at all but for something else?