I really chased my tail looking for the same thing when I needed to scratch build 2 of them for your Liberty Ship kit. I did not find anything you have not except at that time I tried to contact the US Army Air Defense Artillery Museum in Fort Bliss, TX. They apparently have a 37mm in their collection and if anyone had drawings they would be the best bet. Unfortunately at the time I was researching the museum was still in the process of relocating to FT Bliss and no phone numbers were available. It looks like they are open now so a quick Google search should yield some contact info.
Hope this helps.