Seriously doubt it came cleared from the sign company. If the coating was done 50+ years ago, more likely varnish. As yellowed as it looks, more likely varnish. If it was done 30 years ago or less, more likely automotive clear.
You can polish off a coating that is thin and weak, using auto polish or compound with a lot of rubbing time and attention. Not easy.
If you decide to clear over it again, remember that clear actually brings out defects rather than hiding anything. It will increase gloss and possibly stabilize the finish pending compatibility. Most serious collectors prefer not to buy a sign has been cleared.
If you want to re-clear, be sure to prep as best you can for cleaning and adhesion. I recommend using a blend prep cleaning paste like
PPG Specialty SX 1002, available at PPG auto paint suppliers. Use with a grey or white Scotch Bright pad and water. It gives a thorough cleaning and "tooth" for the new coating to stick without easily visible scratches showing through the clear finish.