Rear hub castellated nuts might only be different between swing arm and IRS, but I am not sure (it’s been 50 years since I played with swing arm).
Don’t you have an IRS rear? That should take a castle nut for 68 - 74-ish (unless the earlier one has the same fine threads AND is a lower profile for your application). If it is IRS you might check the nuts on Pelican Parts for a 924 rear - they might be lower profile, too, because they often ran Fuchs.
The only difference I can think of, other than thread pitch or something, is whether early vs later are different heights. Your original photos with the wire locks in place show a regular castle nut - nothing modified there, just the slot cut into the stub axle to accept the wire lock.
I don't have any center cap interference problems with my Fuchs, either front or rear, because I run wheel spacers. I can see where if you have a long-ish drive stub on an IRS rear it could be a problem. “Angle Grinder Time”!