What you have is not missing anything, Singer didn't start putting numbers on the tension knobs until later. The 99 was made from the early 1900's into the 1950's and a lot of things changed over the years. The earlier 99's did not have reverse, and then Singer added the back tack, and the latest had full reverse. One interesting item, Singer never put elaborate decals on the early 99 like they did on the 128 and the full size models of the same age.
You may be able to replace your tension assembly with on from a newer 99, but I don't know for sure if they'd fit.