JdeBPInteresting final note:
#FreeBSD/#NetBSD (and netkit-rwho) #rwhod uses the original "who" UDP port, 513.
There's a "new-who" UDP+TCP port, 550 and a "new" 224.0.2.1 multicast address. They have been in the assigned number list ever since RFC 1090, footnoted to Jon Postel and the "unofficial" BSD "rwho Group".
They've been reserved for 35 years, and I can find nothing to show for the efforts of whatever this "unofficial Group" was.