Tim,
The problems with identifying old cartridges vary with the reason you need the identity. The hardest is if you need the identity for a cartridge collection. There can be several cartridges that have the same or similar general basic identifying dimensions but have different nominal designations. Also, there are cartridges that have different nominal designations, although they are precisely the same as others, because one manufacturer didn't want to use another's name. Subtle differences in case form, rim thicknesses,etc, may differentiate different collectable cartridges. Collath cartridges are often the same or very similar to others. If your interest is being able load ammo for and shoot an old rifle, a precise nominal designation is not necessary; after all, you will not be able to buy it at the local "Mall Mart" anyway. As long as you can find or make cases and bullets that will fit, or at least work, you can call the cartridges anything you want. The "ridge" may be where the chamber stops and the lead starts, therefore may establish the maximum case length, but not the minimum or even the normal case length. Using cases longer than the maximum may result in pressure spikes by failure to release the bullet, this is more important than bullet diameter. If you have cases fireformed to fit the chamber and a way to size the neck, it doesn't matter if you don't call it anything, or if you call it 10.75x65R Collath,or 10.75 X65R LK Express, or even 10.75 Hemmerling. I think you have enough information now to sleep well, especially after you "slug" the barrel. BTY, if you can find a lead ball smaller than 44 caliber, you can flatten it enough to use it for slugging the barrel.
Mike
