Typo was added in Research Unix V5[0] and also present in V6[1]. It isn't in V7, my guess is that it was replaced by spell. I don't think it would be difficult to get it compile on a modern system.

[0] https://github.com/dspinellis/unix-history-repo/blob/Researc...

[1] https://github.com/dspinellis/unix-history-repo/blob/Researc...