This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
From: PeteAlway@aol.com (Peter Alway)
Estes tube numbering seems to have progressed like the numbering
of steps in a BASIC program. The very first Estes Catalog had
numbers unlike the BT-20..BT-50... etc. system they use now.
Sometime in the early 60's they gave numbers 10, 20, 30, 40, 50
to their tubes in order of size. 10, 20, and 30 were almost
identical in diameter (though 10 could be coaxed to fit into
30) All were meant to hold 18 mm rocket engines. BT-10 was
an ultra-light spiral-wound mylar, BT-20 was essentially as it
is today, and BT-30 was a parallel wound heavy-duty tube. BT-40
was also a parallel wound heavy tube that fit over BT-20.
(rather like Quest T-20, but thicker and parallel wound.) BT-50
was as it is today, as was BT-60. BT-5 came along later, I
believe created for the top of the Aerobee 300, and numbered
halfway between 0 and 10. BT-55 came along after BT-60 and was
numbered to fit in. PST-65 egg tubing came after the much
larger BT-70 (originally used just for the Sprite tail ring)
BT-100 and BT-101 first appeared in the 1/70 scale Saturn IB.
BT-80 was created for the Saturn V. It is interesting to note
that two standard HPR diameters, 2.6" and 4" began as scale
model components. Estes also created BT-3 for the Saturn IB,
and BT-51 for the tanks. BT-52 was produced for the BT-60
Semi-Scale Saturn V, and this tube still appears as a
hook-retaining sleeve on some Estes D engine mounts. The
sleeves are still correct for a 1/242 Saturn V. Many BT-5 clone
kits still use the length for an Aerobee 300.