Schreckmeister,
I possibly have some news for you. I received an email from a friend who told me to look at musket M.43 on page 82 of George Newman's first book on Rev War weapons. That musket is also marked 2RT GB on the butt tang. The architecture looks just like yours. It is identified as a German-Dutch musket possibly supplied to German Units fighting for England. The sideplate and triggerguard are very similar. Don't know if this solves the issue but it sure helps.
Al