Personally, I believe the Boxlock vs sidelock was always more a British thing than American. It was definitely an ongoing subject by the time burrard began his writings on the Shotgun, & a subject to which he devoted considerable space. Personally I don't really consider the 1920's as "Recent". I suspect Greener didn't devote much to the subject as his boxlock preference put him in the definite minority in England, yet he really made no great attempt to extoll it's "Superiorty". I will have to get out my copy of the 1909 "Baker Gunner" but as I recall the Baker Co was extolling the virtues of the Sidelock in it. It doesn't really appear to be a Johnny-come-Lately subject to me, but what do I know, I haven't used up a case of linament from patting myself on the Back over my superior knowledge.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra