There are photos of the square shouldered .318 WR in both Hoyem and Fleming's books. I have a couple of observations/questions:

I may be wrong, but it appears that the shoulder on the square shouldered case is slightly further forward, so I can't picture how the chamber could be reamed out from the square shouldered version to the original.

Obtaining, custom dies for reloading would still require standard ammunition to be fire-formed for subsequent reloading. Personally, this is not something I would be comfortable doing with a double rifle due to the risk of rupturing a case and/or worse, although it appears that the owner has been doing this for 20yrs. Therefore, I can see 2 options, obtaining another rifle with the chamber reamed to .318 WR square shoulder to use for fire-forming cases, or get a forming die such as the Hornady in my previous post ($165).

If you cannot get a chamber cast done or send fired cases, surely WR should be able to supply reamer dimensions to the die maker(s) or even obtain dies directly and supply them to Norway?