Saw an early 99 with a rear sight that has a double-side notched elevator. I thought they shipped with a single notched elevator that went in the middle open slot. Would this be a replacement rear sight?

Sounds like the Savage Windgauge 15 that was used on the 1899s.

Marbles also had a rear sight with the double elevator but hopefully it is not that one.

Left side & down is the No 15 Wind Gauge, the early elevators did not have the small tabs that wrap over the top of the sight. All these sights (that I have seen) will be stamped Savage Arms on the top with either patent applied for or a patent date, all Marble's sights should be stamped with their name and King also made similar sights, neither were windage adjustable.

Thanks Mike and Gene.
It is not windage adjustable, so I'm thinking it is likely a Marbles, so that would not be an original sight, correct?

Correct. The Marbles was pretty popular as a replacement.

King, I think, also made a similar looking sight.