Rear axle housing vent

[Lloyd]

I have a hole on the upper part of my rear axle housing on the pass side close to the ring and pinion housing. My book shows it as a vent. Should be a housing vent nipple, a cap and a cotter pin to hold the cap on. I tried to get a bolt started in it today to see if it was threaded, seems to be but not bolt threads - I'm wondering if it is SPT, or use to be, from what I could see the threads are pretty bad. Does anyone know what thread this is and perhaps any ideas what I could put there?

[Don Coatney]

This is what the original looks like on my 1948 P-15. Serves a dual purpose and holds down the rear brake line splitter on the drivers side. If I recall it is 1/8" tapered pipe thread.