New help finding outsize crown for Rotary Aquaplunge

hi Omegoids,
I was at my watch guy today and he passed on some disappointing news. The tube on my rotary aquaplunge is 3.12 mm and he just can't find a crown to fit it. I bought it with a loose crown but that doesn't appear to be original to e watch . Also' the way the case is made as a one piece casting means he is reluctant to go cutting or grinding. At this point I'm not expecting to find a vintage one...does anyone know where I might find a crown to fit such a large caliber tube. Thoughts and suggestions all welcome.
Chips.

Is this a screw down crown? If it's non screw down, then it is unlikely to be 3.12 mm which would be an odd size to me. You can get generic crowns for 3 mm pendant tubes but, they are not common. Or is it an external screw thread?

I did an Aquaplunge last year with a Landeron 149 inside which needed a new crown and it had a (pretty standard) 2.5 mm pendant tube but, there are many variants of this watch. Do you have a picture or know the model number as that might help someone who knows these?

Cheers, Chris