I had specified a short reach Universal 51... ...I think the drop is close to 45mm
The Mod. 51 are indeed 45-56mm and Weinmann 500 are 43-56mm, so it's curious you're having a problem. I'd measure up to confirm (perhaps the forks have shrunk after all these years).
If you do need something shorter and want a period caliper, Weinmann 400 are 40-54mm but for all I know are just as hard to come by as Mod. 51.
Modern calipers are typically 39-49mm, but you'd need to drill the back of the fork out to 8mm to accept a recessed tube nut.