Any factory installed 426 Street Wedge I've seen was stamped as follows:
1964 = V 42 HP (and an assembly date below it)
1965 = A 426 HP (and an assembly date below it)
1966 = B 426 HP (and an assembly date below it) *This year they are a pretty rare piece
All of these were factory rated at 365 horse power and were basically just an over bored 413, same heads, exhaust manifolds, crank, intake, carb size, etc. with some dress up parts like chrome valve covers and air cleaner and a tach drive distributor (that wasn't even connected to a tach). Some industrial or marine applications may not have had the HP stamping.