[cschramm]

You can follow the assignments of such "socket numbers" for NCP in RFCs 349, 433, 503, 739, 750, and 755. RFCs 758, 762, 770, 776, and 790 address both socket numbers for NCP and port numbers for TCP/IP simultaneously. RFCs 820, 870, 900, 923, 943, 960, 990, 1010, 1060, 1340, 1700 continue that work for port numbers only and RFC 3232 finally switches it to an online database.

RFC 349:

        Socket          Assignment
           1            [Old] Telnet
           3            [Old] File Transfer
           5            Remote Job Entry
           7            Echo
           9            Discard

RFCs 349, 433, and 503 also provided a list of conflicting socket numbers used on specific hosts.