The thumb rests were offered on Wes Jordan model bamboo rods and probably other rods upon request. They usually had a palm rest sanded out as well. It also could have just been added by the owner with a file.
Filling it in would work, but look like crap. The best way to eliminate it is either to have the entire grip replaced (the best solution) or just the effected cork rings.
Replacing the entire grip means taking off the reelseat and usually the winding check. Replacing just the effected cork rings can be done WITHOUT removing the entire grip and reel seat but that is usually accomplished by removing just the effected cork rings, cutting the replacement cork rings in half to fit around the blank and gluing them in. That means you will may have a somewhat noticeable line where the halves are joined together.
As far as local repair people go, I am sure most knowlegable rod builders could handle either option. If it IS an Orvis bamboo rod, any repairs done by Orvis will usually enhance or at minimum protect the value of the rod, plus they have the right color thread and other stuff on hand. Every single Orvis bamboo rod I own goes back to them for that reason.
The problem is their bamboo rod shop backlog is VERY long, at present up to a year.
The choice is yours or just fish it and forget it.
Good luck!