DOES ANYONE KNOW HOW MANY DOLPHINS WHERE BUILT, I HAVE A 1979 M-500, BUT IT'S BUILT ON A 1978 TRUCK WITH A 20R, FOR THE 1980 MODEL YEAR... IS IT RARE TO HAVE A M-500 WITH A 20R?, THOUGHT THEY ONLY CAME WITH 22R, WAS THIS A HICKUP IN THE LINE, OR ONE OF THE FIRST MADE SO USE UP WHAT WAS ON HAND, DOES ANYONE KNOW,, THANKS FOR ANY HELP,,,,