Tried to do a little research on the deciphering of the OTK symbol and the various numbers in the rhombus
following the OTK letters.
Basically, OTK is a Russian abbreviation for Quality Control. It is a symbol that is used on a number of products, and it is not specific to military use only. Sometimes the OTK letters are followed by a number, i.e. OTK 23. This simply means Quality Control, inspected by inspector 23.
The numbers within the rhombus could be 1,3, 5, 7 and 9. The meaning of these numbers is as follows (from a Russian seller on EBay):
[size=10pt]
1 - usual OTK
3 - usual + climate test
5 - military
7 - military + climate test
9 - a special series[/size]
Some of my Russian tubes have the OTK symbols printed on them and some not. Those that do not have them probably come from bulk packages that have the OTK symbols printed on the large box they came packaged in. (So far I have seen the numbers 1, 3 and 5 on my tubes.)
My source claims that for audio use the different rhombus numbers do not matter. I would venture to guess that category 5 and 7 may be more rugged, perhaps corresponding to the W (6AK5W) in the US nomenclature, which indicates a more rugged construction for military use.