Companies that made 78s electrically used different systems at different times. This was usually indicated by a code in the dead wax, either as part of the matrix number or in another area. W in a circle was often used for Western Electric, for example. C was used by Columbia to indicate their own system about which I know nothing- was this their own development or licensed from an outside source?
Has anyone noticed a change in recording characteristics after the date this was changed? Does this affect the playback equalization settings?
Has there been a study of these systems, dates, etc.?
If there was no change, there's not much use chasing down some of this data.