In 1913, he would have purchased a Colt Model 1911. That was the earliest Model, unless you wanted him to have a Colt/Browning 1905 that was developed into the 1911....but I think it would be more believable as a 1911. (NOT a 1911A1...)
MANY German officers in World War II carried Colt or other wartime manufactured ones, and wore the US made.45acp autos as a badge of honor, they rather liked them, although MOST of them would have been 1911A1s captured, probably from the British who we sold quite a bit...but some of them carried commercial ones...
And as far as your "obsession" with "magnums," you'd better drop that idea right now
Chances are he and anybody else using them would be using good ol' 230 gr FMJ Military "Hardball...."
First, the word "Magnum" wasn't even COINED until the 30s, and then only to differentiate between the .38 Special and what was essentially "Souped up" .38 Special, which they called the .357 Magnum....
No auto pistol fired ANY "Magnum" rounds until the 1960s and 70s, and none of them were particularly successful either...
The .45 acp, as well as any other .45 rounds of the era, got their "power" from big heavy bullets traveling relatively SLOWLY, but transferring MOST of it's energy to the target instead of traveling through like a "Magnum" probably would...
So go ahead and have him "bounce" around if you have to...chances are, the "big and slow" .45 WOULD do that, while the only Magnum of the time wouldn't do that, because it would probably punch right through in little neat .38 cal holes....