The action of the 7mm looks just like the .375 actions they made. The most important changes are the reduction of the feed ramp to accommodate the longer H&H cartridges and the cutout in the forward ring to assist loading. The magazine has been altered to add more room to the front. There is also a tiny bit more room added to the rear of the magazine by using thinner metal in that area than on the standard action.

In addition to the Whitworth marked rifles, long actions were also used for making Mark-X versions in .375 with US style sporter stocks. If Interarms was making .375 Whitworths at the same time they were making .375 Mark-X rifles, it would make good sense to put Mark-X or Whitworth on the barrel instead of the action. That way they could use an action for either. I have seen rifles so marked.

My guess is that the rifle and action were put together by someone other than the factory. Are there proof marks on the right side of the front ring and the barrel just beyond that? If not then you know it left the factory without being barreled. If proof marks are present it still doesn't rule out the possibility that someone replaced a .375 barrel with a 7mm barrel.

If it loads and feed 7mm Rem Mag properly, and it should, then why worry? You should be able to put the bottom metal into the action without a stock and find out. If you don't like it you could rebarrel or make use of the extra space for a long 7mm cartridge.

If it was mine, I would rebarrel it to .400H&H. I had that done to an early Whitworth and it worked out quite well.