Actually, it is the standard sized frame common to the late actions.
My bet is that it was a 450/400 as you can see that the original barrel shank was larger than the current barrel.
One of the rifles very close to mine was a 450/400 and then rebarreled by Gibbs to 280 flanged.
Mark Crudgington should have the ledger for this rifle so I suspect its original specs can be brought to light. Will look into it.