It is well known that the early Selmer-Maccaferri guitars came equipped with an “internal resonator.” Here is a 1970s CSL copy, complete with the resonator box which can be seen through the soundhole. The resonator had a tendency to rattle after some time of playing, and players usually had them removed. Apparently, few Selmer-Maccaferris survive intact with the resonator.
I have not been able to figure out the functioning of the resonator. The stated purpose was to increase volume, but how did the resonator bring this about? What are the underlying acoustic principles? I have been unable to find info on this, but perhaps some O&M readers know?