When party lines are discussed, people usually talk about coded ringing and that everyone's phone rang with all calls.
However, in city service this did not occur.
The panel dial switching system, developed in 1922, included provisions for two and four party line service, which were common in those days. Each party had their own listed phone number and could be anything, not necessarily consecutive. There was no need for party letters. When calling a party line, the switchgear deteced the type of selective ringing and (bias and ground combinations) and sent out the appropriate ringing current. Only the desired telephone rang. This helped privacy since other parties wouldn't know the phone was in use.
After WW II party line service declined, but postwar Bell System had a bad shortage of capacity to meet new demand, so party lines were forced to continue more than many users wanted. (Movie "Pillow Talk").