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The above Byrne comission page may have been inspired by this Cockrum house ad for X-MEN, that ran during the early Byrne issues of the series.

[Linked Image from i.pinimg.com]

This ad ran in 1978, promoting X-MEN as newly "On sale monthly!"
The last bi-monthly X-MEN was issue 111 (June 1978) and the first monthly was 112 (August 1978), and though Cockrum's last issue on interior stories was 107 (Oct 1977). It might be considered odd that Cockrum was selected to do the house ad.

But Cockrum still continued doing most covers for the series for another 2 years, through issue 126 (Oct 1979). At which point Cockrum resigned from Marvel, and whose resignation letter to Marvel oddly turned up "accidentally" as butler Jarvis' resignation letter in IRON MAN 127 (also Oct 1979), after a clash with Tony Stark in the alcoholism storyline issues.