try clicking the default set and then starting the customize over again, sometime the missing appears when you do that.

Yes, that was the first thing I tried, but no success.

One would expect a re-install of 24.5 would do the trick...............unless, of course, the information regarding chosen options and the current available options are a part of one's default profile?????????????

try a restart with add-ons disabled from the help menu... more likely a bad add-on that anything else.

...or, when you start in safe mode (hold Shift when launching TB), check "Reset toolbars and controls" and click "Make changes and restart".