I hope someone can or will back this up but I was told a while back that confusing proof marks on barrels can often be put down to barrel makers having stocks of tubes on their shelves that may have been proofed in the white and marked with a certain set of marks, then, when they were fitted to an action and made into a gun, they were submitted for final proof at a later date and the marking method had changed.
I'm not sure if it's always the case, may have been for trade guns, I don't know if Midland made their own barrels, or whether outworkers either at Demon works or elsewhere in the trade made them.
I may be completely wrong here, I'm getting more doubtful as I write, but even so, I'd guess that unless a gun had been for re-proof, the latest marks would be the ones to go from.
