RST Engineering wrote:
6V6 was the quintessential beam power audio output tube of the 1940s through
the 1960s. You can generally "read" the missing numbers if there is
anything left of the imprint by gently rubbing the tube on your hair. THe
oil from your hair will stick to what is left of the printing and make it
readable for a short time. You might take a permanent marker if and when
you finally read the number and mark the tube.

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.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com