Double-Sized 20th Anniversary issue. On-sale date from Amazing Heroes #29, with shipping date of 1983-07-19.
This issue is a variant of The Uncanny X-Men (Marvel, 1981 series) #175 [Direct].
There exist further variants:
Letters of comment from Matt Crosby, Monique Barbanson, Gary R. Robe, Alex Patterson, and Christopher Rauls. Includes a small illustration of Professor X, supposedly the writers of the replies to the letters.