I have just treated myself to a lovely, late, coated, 320 mm Aviar. It's a gorgeous thing - TTH machining was just so slick - but it lacks a flange. I have a universal iris that will hold it but I am reluctant to risk mucking up the threads and the finish by doing so too often.

Does anyone know of a good source for imperial-measure standard rings? The Lens Vademecum says this lens needs a 3" flange. Ideal would be an original TTH flange (they were proud of their thread-starting profile), but any blacked brass ring would do.

My standard source for thread gauges is on holiday, so in case I need to get a flange made: what is the thread pitch on these flanges?