Hi Tim.
The so called notch is a relief cut in the top front edge of the receiver to provide clearance for the rear sight when in it's lowest position. Many unused Artillery barrels were left over when the war ended and some "Commercial Artilleries" were assembled using these. Even more were produced by Mauser using these surplus barrels for their Siamese and Persian contracts in the 1930's, and a few were even assembled in 1946 when the Mauser factory was under French control. I don't know which, if any, of these Artilleries produced after 1918 had the "notch".
Regards, Norm
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