Published by Marvel Comics Group, Volume 1 was published continuously from issue #1 to issue #66 (Mar 1970), when publication paused some 9 months, the title resuming as reprints of the earlier issues. As of issue #94, following publication of Giant-Size X-Men #1, the title returned to original content, featuring the new line-up of X-Men, and continued until issue #141 (Jan 1981), before changing to Uncanny X-Men (1981), continuing with original numbering.
Although always distributed under the Marvel Comics Group imprint, the Publisher of record in the indicia has changed several times in its history:
Issue #'s 1 - 50: Canam Publishers
Issue #'s 51 & 52: Perfect Film & Chemical
Issue #'s 53 - 80: Magazine Management
Issue #'s 81 - 141: Marvel Comics Group
The early publishers are a vestige of the transition from Martin Goodman's ownership as the modern-day Marvel Comics arose from its Golden Age roots in Timely / Atlas.