Horn wiring

If Redfish is correct and the horn wiring scheme is the same as '67 then:
Power to the horn relay center terminal is from the alternator output stud.
and therefore B stands for Battery, or Batt terminal (which is what a Chrysler alternator would have marked on it)
The relay terminal perpendicular to the power terminal goes out to the first horn.
The relay terminal with the S gets a black wire to the bulkhead and then to the horn switch.

This is for 67. '63 colors may be different but relay looks the same, and the operation is the same.
H3-18 BK* translates as Horn wire 3, 18 AWG, Black with tracer.
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'63 bulkhead connectors look somewhat like '65. Best bet for which wire cavity is the black from to the horn is a '63 FSM.
-63 had no fusible link. Be extra careful when working with the main circuit.

That's a GM SI alternator. Don't know what the wires in the wire nut are.