Another thing that has worked for me is to put the tube in the freezer
for a few hours. Frost forms in a different pattern around where the
number was than on the rest of the envelope. Sometimes.

And after you take it out of the cooler, "huff" on it once or twice
(just as people "huff" on their eyeglasses before wiping them with
a cloth) and the moisture from your breath may outline the tube
identification.

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license