When I bought my 66 mustang it already had flowmasters installed with true duals. How do I tell what model flowmasters I have?

 

You should see some numbers stamped on it I believe.

Use this as a kind of reference:

430402 means...

4 = case thickness (4")
30 = inlet diameter (3.0")
4 = series designator (40 series)
02 = inlet/outlet locations (center inlet/dual outlet)