The description of Spectators on page 30 of the Monster Manual begins:
A spectator is a lesser beholder that is summoned from another plane of existence by a magical ritual
Which plane do they originally come from?
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The description of spectators on page 112 of Monster Manual II for AD&D 1st edition says:
Spectators are summoned from Nirvana [...]
The description references the spectator returning to Nirvana several more times in the text (e.g. if it is not commanded to guard a treasure, can no longer fulfill this duty, or is released from service).
Nirvana is another name for Mechanus. (The plane was simply called "Nirvana" in 1e, but was referred to as "Mechanus" in 2e; from 3e onwards, its full name was stated to be "the Clockwork Nirvana of Mechanus".)