About "First come, first served."

There is more to this than met my eye.

Come and served are past participles used here to create noun phrases.

First may be an adverb:

First come = those who are come first.

First served = those who are served first.

Or first may be an adjectival noun (as used for example in the saying “The first shall be last and the last shall be first”).

First come = first {of those who are} come.

First served = first {of those who are} served.

However you choose or combine these interpretations, you end up with two noun phrases that are in apposition, suggesting their equivalence.

Interestingly, the use of the past participles is not strongly tied to the past. The participles as used here seem to relate to the past, present or even future state of those who come or are served.