So, my k5 has the #13 in yellow/white chalk on the firewall, this # just became visible to me after I pressure washed it. It also has the #640 in red (like a lipstick red) that's more visible then the #13.
It's original 305 tbi I just pulled out. In the cab it has holes from a light bar, like the ones on cop cars. The original color is green, like a Arizona Border patrol green. The head lights sometimes flash when turn signals or flashers are used.
Basically I'm wondering if anyone knows what the numbers mean or have found some on there vehicles?

 

Sounds like the type of numbers parts yards put on items. Pretty sure they aren't from the factory.

 

Like AJ said, sound like something a parts yard would use.

As for the flashing headlights, I would check your wiring, because that does not sound normal, even if it was something related to a border patrol. All of the electronics involving that would have been removed.

 

Some vehicle auction yards also use paint stick under the hood for the group lot# and the # the vehicle will run thu the sale - so it may have been a border patrol unit at one time and was sold at auction