The classifications of central offices in those days were "magneto," which was understood to mean magneto signalling, local talk battery, and "common battery," which was understood to mean common talk and supervisory battery and machine ringing. This was the usual teleco usage as applied to central offices.
The central office had machine ringing whether it was magneto or common battery*.
A very significant part of common battery operation is the supevision that was inherent. Local batteries would have been incompatible with the supervisory functions. The same battery was used for both talk and supervision.
But there indeed might be some magneto lines in a common battery exchange. That was particularly true in earlier days when rural magneto lines (usually one conductor and ground) were served from a common battery office. And, yes, there might be 20 or 30 or more parties. They were in variably very noisy and also other subscribers might listen in, creating an additional source of leakage.
Customers on these lines had code ringing and magnetos to signal the operator or other subscribers on the line. These terminated on a different section of the switchboard, where either typical megneto drops signaled the operator or adapters brought in a lamp signal. Either way, while customers were supposed to "ring off," when they finished their conversation, many of them failed to do so and the operator had to challenge from time to time.
These were usually owned by the subscribers and also maintained, or not maintained, by the subscribers.
Common battery operation required pretty clean loops. Magneto lines could operator in very marginal conditions.
Very long loops could not supervise in a common battery environment, so there might be some exceptionally arranged for magneto (ringdown) signalling and which required local battery at the distabnt location.
Different, and not exacly comparable to central offices, were toll stations and private line circuits used by "right-of-way" companies, such as railroads, pipeline companies and others. In the days when Bell companies and most independents would not connect with non-telco opeerations, "right-of-way" companies were an exception and telcos would interconnect with them freely.
- - There were reportedly some very small magneto exchanges where the operator had a magento crank she turned manually. I never was in such an office.
Wes Leatherock snipped-for-privacy@yahoo.com snipped-for-privacy@aol.com