I did a bit of investigation and verified that the air tube fitting threads are BSPP, not BSPT, That is, they are a straight thread, not a tapered thread. They are J.I.S., but J.I.S. is a direct copy of British standard threads). I checked in McMaster-Carr and they have various plug forms in BSPT and BSPP in brass, 304 stainless, iron & steel. They cost about $2 each. So here's what I plan to do. I plan to use a BSPP hex key plug, threaded all the way to the bottom of the exhaust port. I'll apply a bit of high-temp thread sealant to assure a seal. While this would be adequate, (my being a belt & suspenders kind of guy), I'm also going to use a BSPP hex head plug to cover the hole. I also came-up with an alternate approach: I tried this on one old fitting just-for grins... Looking at the tube fitting, I found that the tube hole on the manifold side of the fitting is 0.203" dia. This is the perfect size for a 1/2-20 tap. I tapped the inside hole of the fitting from the manifold side with 1/4-20 threads. I coated a 1/4-20 x 1/2" threaded socket head set screw with high-temp epoxy to help "seal" the threads and inserted the set-screw into the fitting. I then used a moto-too to grind the tube sticking from the hex-head side of the fitting flush and dabbed a bit of epoxy on to look more "finished". I'm not going this route because the fittings are quite rusted and the hex heads are rounded, but it is a viable alternative.