Comma before "in 1973"

Question

At one time, in 1973, Winston had held in his hands evidentiary proof that certain people who the Party deemed never existed had actually existed.

Is a comma required before "in 1973"? Because "in 1973" looks like essential information to me.

Answer 1

The comma changes the meaning slightly.

Without it "in 1973" is restrictive - it is defining the time as being in 1973, so "one time" selects a time within 1973.

With the comma, "in 1973" is non-restrictive - it is commenting that the relevant time was in 1973.

Practically, there is probably very little difference here. But if, for example, the reply was "No, that was in 1975", then in the first case the reply would be directly contradicting the sentence, but in the second case it would be quibbling over a detail.