I would definitely try cleaning first before you try to adjust anything. A lot of the times it is just a matter of gunk and filth clogging up the mechanism, and from the picture, this would be my first route. Clean first; and after it is clean, maybe a touch of oil. If that fails, then move on to other troubleshooting.

The area where the tabulator brake (a piece of cork or leather)  that pushes up, underneath the rack frame, is either dirty, rusty, or both.  This area will need to be cleaned or sanded.  It should work after this.  Make sure the underneath the rack frame where the tab break lifts up on it is smooth.  If it doesn't lift it up high enough to clear the escapement wheel pawl gear, it will need to be adjusted.  Underneath the right rear top cover (with the machine facing you) needs to come off.  Engage the tabulator and you'll find the bellcrank that has one or two screws in it.  This is the adjustment you'll need to make.  You"ll want to lift up a little on the tab brake after you loosen these bolts, tighten them, and then let go of the tab brake.  This should help it, unless you've overadjusted, then it will need to be backed off of slightly.  It's trial and error to adjust, but not prohibitively difficult.

I had a look for this adjusting screw, but all I could find was the pivot screw as you can see in the first picture and the pivot screw for the mechanism that engages the tab stop. When the tab key is pressed it allows the carriage to move freely back and forth. So the part you can see circled in the second picture is able to engage properly.

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When I press the tab key the tab mechanism moves far enough to allow the carriage to move freely but not far enough to engage the tab stop.

I could see an adjustment to the tab key linkage, but it is already adjusted all the way and this is not far enough.

Any advice would be greatly appreciated.

I am happy to announce that I have solved the tab problem and now have a working tabulator. What I did was remove the rear right cover and the right side glass panel.  Then I removed the bell and removed the bolt holding the tab mechanism to the frame. This gave me enough room to remove the adjustable linkage between the tab mechanism and the tab key.

I cleaned up the threads in the linkage and backed off the locking nut all the way. Then I reinstalled the linkage making sure to screw it all the way in. Now the tab key releases the tab break enough to allow the carriage to move and moves the mechanism far enough to engage that tab stop.

As an added bonus this adjustment also fixed the margin release.



In this picture you can see the linkage adjustment and where it connects to the tab mechanism.