Thanks Allen, I wondered about that keyhole vent pan also but didn't get anywhere. What you bring up is an interesting possibility, maybe Richard will chime in as he's the Ruler of the ReallyOldStuff around here. Maybe I'll look in my copy of Essenwein and see if there's anything like it pictured there.
Great drawing, by the way, Mr. da Vinci. BTW, how come you aren't working as a commercial artist?