No secret LarrBeard. Just as you suspected a 1/8" pipe thread plug with a cap loosely stamped on it. When we sold axles to Jeep in the latter years, that vent was almost always attached to a hose and directed up to the body so water fording would not invade the axle. It is a very good idea to duplicate that. When I worked with the off road guy's that mod is mandatory. The only time I remember issues like this with our axles was after high RPM usage or, if the vehicle sat significantly inclined to the side allowing lube to fill the tube. The pressure produced in the axle is really more atmospheric than anything. The vent helps relive pressure on assembly. Like when you push in the axle shaft on the sealed unit. It never should come into play. Somehow that tube got filed and the shaft slung the lube against the vent. The axle shafts are so rough they do throw a lot of oil.