The phone dates back to a time when all phones were professionally
installed and jacks were not used. Connect the line to the L1 and L2
terminals on the block. This assumes that the phone is still wired
conventionally. For more information see
http://www.porticus.org/bell/pdf/502-521-410.pdf.

These phones were always leased prior to the 1984 Bell System
divestiture. The insides of them were recycled. From your description,
the phone was originally manufactured in August 1958, and was
reconditioned in August 1975. This was in the early days of modular
connectors and the reconditioned phones were being retrofitted with
modular plugs.