The D1ZX is the '71 Boss balancer. The D2ZX is the '72 HO balancer. People try to sell the D2 as a Boss but if you are restoring a '71 Boss you want the original production line part, the D1.
As near as I can tell, there were a number of '71 Boss parts that were only available on the original production enginge. The balancer is one of them.
The D2ZX functionally is the same balancer but is a substitute for the D!ZX. To a restorer it is not as valuable and it was the Ford Service part. It's much more common yet still rare.
There are other aftermarket substitutes now but for looks alone, the "Boss" is the heftiest looking.