CLUE #80 - FOUND!

CLUE #79 - FOUND!

CLUE #78 - FOUND!

Update: I've determined that the current clue is too hard, and is also missing a piece. 

Here is a key:

xXXXx = o

Also, between lines 4 and 5, this is missing: XxxXX

Hope that helps!

CLUE #77 - FOUND!

 

 I'd like to thank the Elsewhere Philatelic Society for installing this.