Filter Adapter for Ilex Acutar - 215mm

I have an Ilex Acutar - 215mm. I need to get an adapter/adapters to fit my filters to it. It is not threaded so none of the threaded adapters I have will mount on it. The outside diameter is 50 mm and a 50 mm slide-on adapting to a 52 mm thread would work. If such a thing exists. I can find a lot of threaded adapters but no slide-ons.

What next?

I wonder if SK Grimes might have something? Also, I know Cokin and Lee make some kind of adapter for non threaded lenses, but not sure if they'd work for threaded filters, as opposed to their square filters.

After posting this I ran another search and turned up a thread from early this year that referenced the Series filters. That led to a source on eBay - macintoshcat