Now I'm really starting to be glad you brought up this topic, Don. I just looked in the '67 manual
that JD posted in the Yahoo Interceptor Group files.
"After the first 500 miles, and subsequently, about every 2000 miles, the tank, sump, timing case and felt filter chamber should be drained. Drain the tank and pump by removing the two hexagon headed plugs on the timing side and the two round headed plugs on the drive side of the crankcase and oil tank. See Fig. 2."
This file is Page 4 and 5. The drawing "Figure 2" shows the two round head plugs. The nomenclature in the parts book is totally wrong - not chaincase but crankcase(/oil tank) and not level but drain plugs. The answer to "why two?" is that the oil tank chamber is separate from the crankcase oil chamber.
Thanks for causing me to look into this, Don. My next oil change will include draining these and the timing case.
Is there a shortcut to draining the timing case that doesn't involve removing the entire timing case cover?