How do you know it's to short. Did you try to replace it?
Looks like a NR3 to me. Look on the other side of it, against the wall.
use a mirror.
I have seen some in the past that were actually a 3/4 prv that had 1" union flanges. Not sur eif it was this model. could have been the wilkens 70 DUC. Could be the 70 and the NR3 are the same length and union