In article k.net, "KØHB"
writes:

"N2EY" wrote

There was an ARRL HQ station before W1AW. What was its callsign?

----- W1MK

You are correct, sir! And before that, it was 1MK

There's a second callsign for the ARRL lab. What is/was it?

----- W1INF

"It's Never Finished" - again correct.

More trivia: There is an IARU HQ call sign. What is it?

That's easy: NU1AW

While it may
have a certain familiarity about it, especially the suffix, what is the
significance of the prefix?

The original system for licensed amateur calls was a number and two or three
letters. Maxim held 1AW, for example. This worked fine until amateur began
working internationally and there was no way to tell what country a ham was
in. When the first shortwave QSO was made by in November of 1925, the stations
involved were 1QP, 1MO and 8AB - the last one being in France.

So amateurs invented the idea of unofficial prefixes. "NU" meant "North
america, United states". So while Maxim's call was shown on the license as 1AW,
on the air he would use NU1AW to indicate where he was. Other countries had
different prefixes, all according to the unoffficial system

To emphasize that the prefixes were unofficial, they were usually written lower
case: nu1AW

The situation was finally sorted out at one of the radio conferences of the
'20s (1927, I think) and the USA decided that American ham calls would all
start with W (in CONUS) and K (outside CONUS).

73 de Jim, N2EY