AT&T System 25 - Manual (Page 39)

INSTALLATION
Symbols Used in Figures
Modular jacks are shown by the triangle symbol. The 25-pair connectors are
indicated by shaded blocks. Generally, only one leg of an octopus cable is
shown. Unterminated wiring requiring cut down or other termination does not
show symbolic designations. The 103A Connecting Block is a typical
modular wall jack that provides cut-down connections for building (station)
wiring and a modular jack for connection to terminal equipment.
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