While it is true that C1 was issued in December, while C3 was issued back in May, Scott does not immediately assign catalog numbers. Also, Scott will reserve catalog numbers if they know or think that a set will expand. I do not know the exact mechanics of how Scott made the assignments, as I do not have a 1919 Scott catalog.
However, I can tell you that back then, the airmail stamp was NOT considered back-of-the-book. In fact, those first 3 airmail stamps were actually assigned #520-522, as part of the normal postage listing, in part because those stamps were valid as ordinary postage. Other "BoB" stamps were allocated larger catalog numbers -- for instance, official stamps started at #1500.
Scott realized that the catalog numbers allocated for ordinary postage stamps (#1-1449) would eventually be exhausted. So later on, Scott did a major restructuring of their catalog numbers, and introduced the "prefix" system, where a capital letter prefix was used to denote semipostal, airmail, special delivery... stamps.
When they did that, the old #520-#522 was shifted to #C1-#C3. If you look at a modern Scott catalog, you will see that #520-#522 are "missing".
Well, that's probably more than you wanted to know. But, too bad.

k