Comma or no comma: "...save for the last digit [,] which is 2."

Comma or no comma: "11112 is a number whose digits are all 1, save for the last digit [,] which is 2.

My sentence is of the following form:

Comma or no comma: "11112 is a number whose digits are all 1, save for the last digit [?] which is 2.

Does the place with a [?] require the use of a comma, require that there is no comma or a comma is optional? I’ve read that a non-restrictive clause requires a comma before which but I’m not sure if this is a non-restrictive clause (I just encountered this terminology today) because all examples I can find have the non-restrictive clause in the middle (My car, which is pink, is ugly.). Moreover, I would like to stick to the original sentence with minimal modifications.

Explanations and references will be appreciated!