The M20 and M21 started using the 376 carb in 1955. Assuming you have a 1955 of later bike, the correct carb would be a 376/21. The carb you have 376/30 is from a 1955 or 1956 Panther 100. The only difference between the two carbs is the slide cutaway.

A concentric will certainly work if that is what you want.