The paper table folds back to show this bar, which has slots on its front and back to hold the metal stops. When folded forward the paper table rests on the top of the stops so they won't fall off.
As you may have noticed sometimes happens, Plan A, to use slotted washers cut in half, looked likely, and easy to do, but failed because the slots were too wide. That meant the stops wouldn't stop.
Plan B was to make the stops from a strip of 0.032" x 1/2" x 12" K & S brass from Ace Hardware.
Step one - make a slot that would fit on the tab bar.
Two small cuts with a junior hacksaw leave a tongue that breaks off by moving it up and down with a pair of thin ended pliers.
The slot this left was filed more accurately square with a flat needle file, and tested with the typewriter bar from underneath at the back until it just fitted.
The end of the strip with the slot in it was cut off to leave the n-shape, and the top corners rounded with a file.
With two of these in place the tab button has a use. If you need more, make them the same way.
Using the tab stops makes the carriage stop suddenly and it was sliding on the oak platform of the typewriter stand, so I put a piece of non-slip shelf liner under it. I'll cut it to fit in due course.