Saving Old Seagulls

On the 102 AC and similar models there's a wing nut which varies tension on the tiller arm, by courtesy of a rubber insert inside the tiller arm lug's conical bore. For now, I've replaced the wing nut with 2 x plain nuts from the junk box, as the original wing nut had it's wings busted off. I'd like to source another wing nut, stainless preferred - does anyone happen to know the correct size and thread ? Looks coarse, maybe BA ?

Thread size is 3/8" BSF. The original wingnuts were brass stampings which have a nice period look to them, usually with a 'No.3' size on one wing of the nut. I haven't ever managed to source replacements.
I did find a couple at a steam rally a while ago which were tapped to 3/8" Whitworth, which I brazed up and drilled/retapped to BSF thread.