No worries.
I was thinking the same thing regarding a rebarrel. The "1"s are different in the serial number.
Could have been done and as the magazine and bolt face pretty much preclude converting a 280 into anything else, it would stand to reason that a shot out barrel would be replaced in the original cartridge.
I am not sure if a smith would go through the trouble of replicating the "Cal 280" script on the barrel though.
Anybody know if Mauser themselves ever carried out rebarrel jobs or supplied replacement barrels for customers? Could explain both stampings...
Also, regarding the lack of British proofs, I would think that there would be some evidence of the British view mark on the receiver ring too if it was a Brit rifle. That can be covered up with stippling or such but there isn't really a reason to cover it up.

I will just have to shoot it and see how it does.
The question about bullets stands though.
I'm half tempted to try .284s first because I don't want to run into the (scary) problems I had with the Gibbs. If it groups with those I would probably stick with them since bullets are so much easier to find.

As to the question about the mag length not allowing the 160gr pointed bullets, I have read that the original load used a 140gr blunt hollow pointed bullet! No wonder it got a bad rap!
This rifle was made just before WW1 so maybe the heavier, longer bullet loads weren't around yet. That could also be the reason that the 280s are usually found using magnum length actions, Lancaster being one maker that used the standard action for their 280s.
I will have to have my brother pull out the Lancaster to see what the proofs tell us on that one.