WVFRED:

I looked thru the book and did not find any reference to the bolt handle on your rifle as a standard option. Bolt knobs on many Mausers have been flattened on the bottom, that is common both in military and civilian production, maybe the most commonly seen civilian rifles with this feature being the FN actions sold under the FN and Husqvarna names, among others.

Two separate rifles with three pictures of top-flattened bolt knobs are included in Speed's book; page 267 and page 429. The flattening is not exactly like yours, and both of these rifles are vom Hofe Magnums. Unless I missed one, no picture of a British Mauser included in the book shows a top-flattened bolt knob.