Know that I think about it, the rimless was 6,5x54 Mannlicher-Schoenauer but the rimless was actually called the 6,5x53R Mannlicher, note the '3". The proof marks might show 6,7mm or the like which is just a land/groove thing, it doesn't mean (in itself) that you have an odd bullet diameter.

You asked how good the caliber was/is; when I visited Holland and Holland nearly 25 years ago, they had several old Mannlicher-Schoenauers for sale. They said they expected to have a lot more for sale shortly as the British government was about to introduce new "minimum energy" requirements for cartridges and, for red deer, the 6,5 Mannlicher fell short. However, as H&H staff pointed out, that along with the .303 British, the 6,5 Mannlicher had been an extremely popular cartridge and had probably accounted for more red deer in history than any other cartridge made. And that's not to mention all the countless African game it killed, including a great many elephants. So it was a nonsense law.

Because of the very long pencil-like 159-grain bullet, the standard for this caliber, it has great sectional density and for large game this makes up for any perceived problems on paper with lower energy. RWS still make this bullet and still load the 6,5x54 M-S with it but, if you reload, the 160-grain Hornady and 140-grain Nosler as well as 125-grain (? brand) bullets are all accurate in my 6,5x54 M-S carbine. Considering the distance the smaller bullets have to jump to the lands, this comes as a surprise. There's no reason why you can't use smaller bullet weights either, especially if these regulate better in the cape gun to the original sights.

The proof marks may also indicate the bullet weight the rifle was originally regulated for. Sometimes, you can narrow down what the rifle is probably chambered for, using this information. I was shown a 6,5 break-open "Tell" rifle that I was told was "probably" a 6,5 Mannlcher although brass cases didn't fit it's chamber. How they came to that conclusion then, beats me. After looking at the proof marks, I dropped a 6,5x58R Sauer into the chamber and the action closed perfectly on it.