What is the significance of the CONT # on the tube of my PVS-7D?

Basically tells you how the tube was paid for. Contract numbers are useful in determining when the tube was produced and what the min specs were for that particular contract. They will sometimes give you information as to what branch of the military the tube was made for and even what type of scope the tube was intended to be used in.

Google the contract number and many times lots of information can be found. Very useful information to have.