Half-Life wall writing

[Boogyman93]

I was just playing the SP and I saw some blueish lines on some walls. When I looked closely I was able to make up something.
This is the original image.

And here's what I think says there.

Is that the texture name?

 

[sub]

I think it says R-8 13b

 

[J-Dude]

Yep, I've noticed that before.

 

[Boogyman93]

Why would Vavle put that on the walls anyway? Is it a easter egg?