I think that the ones before 1925 could have been recorded on an Edison wax cylinder, cutting them by using a stylus driven only by mechano-acoustic power amplified by focusing sound waves using those big metal horns. After 1925, electronic amplification was available. Call this an educated guess...I may be wrong
acoustic vs. electronic
I recently bought a 2CD set of Bellini opera arias. Those before 1925 are on one and labelled acoustic, those of a later vintage(1925-1949) on the second CD are labelled electronic. My guess is it has to do with the microphone recording the various artists. Am I right? Any replies appreciated.