AB, and the logo it sits in are trademarks of the Allen-Bradley company that made the part. ANy brand will do if it meets the resistance and mechanical criteria for your application.
The resistance is 2Meg - 2 megohms - 2,000,000 ohms.
If Barcus Barry made the thing, contact them and ask if they still have such a part.
Can you post a photo of the bad one? I expect we may find something similar at a supplier like Mouser or Allied.
If the 1/2" dimension is important, try looking for 12mm instead. Most pots are dimensioned in metric units rather than inches, though I am sure there are exception.