Rich Brazell wrote:
I did some research on my gear (Ken Knowles or Phil Tucker) and discovered it had "vent holes" to allow the gas to escape during the welding process .  I can still see them today .  They are very small , maybe #45 drill ?  I also read somewhere that they should be filled with oil and caped after welding and heat treating .  Maybe a very old NL is where I saw it?  It may be possible that condensation is creeping in the holes and causing the "stains ?"   Don't take me to the cleaners regarding the filling and capping !   

   A syringe of "Tito" into the micro holes may do the same thing as oil ?   
 
 RB   

I was led to believe that the holes were for adding linseed or Lion-oil to the inside of weldments like the landing gear & motor mount for corrosion protection. That's what Ken Brock told me. You squirt it in with a syringe, spread it all around, then drain out as much as possible. Finally you plug the holes with small drive screws. Two birds with one stone... pressure relief when welding & convenient holes for adding corrosion proofing. Anyway, plug the holes if you have them. 
Paint your gear black - you won't see whatever that is, and you can't as easily see the cracks that may develop either!  

  BEEP!