This issue has variants.
Letters page comments by John Byrne, replies by Chris Claremont.
in X-Men : l'intégrale (Panini France, 2003 series) #1977-1978 (mars 2004)
in Classic X-Men (Marvel, 1986 series) #15 (November 1987)
in The Official Marvel Index to the X-Men (Marvel, 1987 series) #5 (March 1988)
in El Asombroso Hombre Araña Presenta (Novedades, 1988 series) #63 (Junio 29 de 1989)
in Marvel Masterworks (Marvel, 1987 series) #12 ([August] 1990)
in Essential X-Men (Marvel, 1996 series) #1 (1996)
in Marvel Gold. La Imposible Patrulla-X (Panini España, 2011 series) #1 - Segunda Génesis (Abril 2011)
in Atlantic special (Atlantic Förlags AB, 1981 series) #6/1981 (1981) [original title: Armageddon Now!]
in Classic X-Men (Marvel, 1986 series) #15 (November 1987)
in El Asombroso Hombre Araña Presenta (Novedades, 1988 series) #63 (Junio 29 de 1989)
in Gli Incredibili X-Men (Edizioni Star Comics, 1990 series) #25 (Agosto 1992) [in this form]
in Marvel Masterworks (Marvel, 1987 series) #12 ([August] 1990)
in Essential X-Men (Marvel, 1996 series) #1 (1996)
in X-Men Classic (Marvel Italia, 1995 series) #4 (Gennaio 1996), #4 (Gennaio 1996)
in X-Men Forever Giant-Size (Marvel, 2010 series) #1 (July 2010)
in Marvel Gold. La Imposible Patrulla-X (Panini España, 2011 series) #1 - Segunda Génesis (Abril 2011)1st Byrne and Austin issue of X-Men.
Sources for particularization of letters credits are The Marvel Comics Index (Olshevsky 1981) and The Official Marvel Index to the X-Men (Olshevsky 1988).