This issue has variants.
The on-sale date is the publication date reported in the U. S. Copyright Office filing.
Marvel consistently credits this cover to Marie Severin and Sal Buscema. The original art was signed Sal Buscema (info submitted by mikebo via the GCD Error Tracker). Buscema fully rendered a Severin sketch for the cover of #69, so it seems likely that was the process here.
Before the discovery of the original art, we had previously considered several possibilities: Sal Buscema pencils and Al Milgrom inks with the Storm figure penciled and inked by Dave Cockrum per Steve Whitaker; Dave Cockrum pencils with Sal Buscema inks (with alterations on Cyclops, Colossus, and Professor X by Romita) per Nick Caputo; Marvel's Official Index to the X-Men #2 gives the penciller as Marie Severin and the inker as Sal Buscema; Sal Buscema pencils and unknown inks per the original indexer.
1st time Wolverine goes into a berserker rage. 1st mention of Storm's claustrophobia.