After I posted the comment, I ran into Ken St. Andre's blog and he had a post about the calendars. Mentioned that he thought the T&T calendars were probably the first fantasy gaming calendars on the market and stated the first one was 1978. But I don't know if he meant the 1979 one or if there was a 1978 release.
Couldn't find any solds for any of the calendars and no one selling one. (You had probably already looked yourself though). I collect Pin Up Calendars, and can say that in the last 10 years THAT market has really taken a beating - I picked up a bunch last year in shrink for less than they cost to ship. The same ones that 15 years ago easily brought $30-50 each, open but in good condition. What you have is a completely different thing but if you were trying to sell - you won't have the interest from 5th Edition or Stranger Things driving up the price. I could be completely wrong about that though - you probably saw that a pair of early T&T books pulled ~$1200 not that long ago. So I guess I'm not a lot of help
On a positive note: Since that's a leap year calendar, you can use it again in 2036
