I was working in Broadcast when "XLR's" came out. I don't recall any
reason for the designation.

As I recall, they first appeared on non-broadcast mikes from
Electrovoice and Shure as a better smaller answer than the abominable
screw-on cheapos previously used. The EV654 was the first mike I had
with XLR built into the case. Still have it. It's not good for much!

The 'standard' in the 40's was (As I recall) The "M" round 3-pin
connector about 1 inch in diameter, as seen on the old RCA OP4, OP6 and
OP7 Remote Amplifiers. The Gates elcheapos had the screw-on type. (What
the heck were those called??) I have and old OP-4 and a Gates remote
amplifier.

In the 50's the 'new standard' was the "D" shaped Cannon "UA" with gold
plated pins. It has a real positive locking mechanism. The first piece
of gear I ever bought with them new-fangled Transistors in it was a
Collins remote amplifier. It came with "UA" type connectoirs, and by
1960 we had lots of cables with male UA on one end, that plugged into
the Collins, and out in-the-studio-wall mike connectors. The other end
was female XLR for the 'cheap mikes'. The 'Good Mikes' like RCA 77DX's,
had their own cable connected inside the mike case, like It Should Be.
RCA didn't change that when the BK-5 and BK-5B mikes came out in the
60's. So you had to put your own connector on the cable end of a new
RCA mike.

Jeez, who the heck wanted to know about this?? Sorry...