It's been three years more or less since I made this chart, and we've learned a lot since then, I plan to update this soon, so don't take this version as gospel. For instance, the first vents were blank, and the 76/77 years will be divided. Might be a few other minor tweaks to make as well. We should now have photos of every vent so I'll also make up a poster using actual vents, in addition to this type. Stay tuned, and be patient.
Marty