That gun does look post war - especially the bolt handle. Not to say the bolt couldn't have been changed in the interim.
The Rigby records which are reproduced in Speeds Mauser book show many 416's with Mauser serial numbers from years prior to the Rigby made/sold dates. One was made in 1929 with an action from 1912. There is also a Rigby chambered in 505 Gibbs from 1924 on an action made in 1901!

I suspect that if you owned a magnum Mauser action or complete rifle post war (or pre war for that matter), then Rigby would have been happy to build a 416 on it for you.
I also suspect that if Steve Barnett had solid info placing this rifle pre war, then he would be asking much more than he is...