The Sun Type 3 keyboard labelled its meta keys "Left" and "Right". The Type 4 changed this to a hollow diamond, or lozenge: ◇. The Type 5 and on uses a solid diamond: ◆.
My question is: why use the diamond symbol for meta?
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To elevate Patrick Schlüter's comment into an answer: here's a picture of one of Sun's keyboards:
Compare and contrast with one of Apple's keyboards:
Or even with a modern Microsoft keyboard:
Sun's diamond symbols sit where Apple uses the Apple logo and Microsoft uses the Windows logo; it's therefore a safe bet that they reference Sun's diamond logo as shown on the top right of the pictured keyboard.