Many thanks Bill, but it's a little smaller than 5/16". I've dropped a micrometer across several original examples now, & apart from one that looks to have crushed threads & which measures about .285, all of the other three or four are consistent to within a thou at .295/.296" (diameter measured across the top of the threads). My 9/32" ME dies arrived today & it doesn't appear to be either of those. It's quite a fine thread. I'm stumped at the moment.