I hate to put it this way, but there really isn't alot of detail in the drawing to distinguish it from one make or another. Most likely it was drawn as a " generic" car, or a "simple" version of someones car.
As ffuries stated, most likely initials of the owner.
Cars of that era did not need keys yet. Half of the population had not even seen a car in person, let alone know how to carjack it