Replaced the piston & nut assy for the GE GSD2800D03 with the replacement part WD24X10018 which GE indicates fits all 3 model subsets: D01, D02 & D03. Unfortunately the replacement part has a piston that is too short, it is 1 3/4" in length and the old piston in the D03 is 2 3/4" in length. Can someone provide the correct part number for this Piston & Nut Assy which is 2 3/4" in length. 
What problem is it causing?
I looked at a number of different machines in the GSD series and the Piston and Nuts all looked the same.
Blocked Washer Arm. The new piston by being an inch shorter does not seem to actuate the flap above it. ??
I am sorry you have lost me.
Here is a GSD2800G03 parts breakdown, this area of the machine should be the same as yours.
Replacement parts for GE GSD2800G03 | AppliancePartsPros.com
As far as I know there is no flap.
Also I cannot see how it can block the washer arm.
What I should have said is that with the shorter piston assy the “message” “blocked washer arm” is indicated. I spoke with GE parts this am and the original part was supposedly wd24x199 (which I assume is the part I was trying to replace" The piston being longer. I cleaned up the old piston assy and the washer works without warning beeps but when I install the new one receive warning messages. I don’t think my old piston assy is going to last since it already looks worn. There is a flap on the screen assembly that resides over the piston assembly on the filter piece. I believe that when the piston is fully pushed up it hits the flap on this filter piece.
Yes I took another look at the parts breakdown and the shaft does look like it is extended in the drawing but there is no shaft showing in the actual picture.
Looks like this part has been updated and from your description the screen/flapper must also have something done to it but I do not know what that something is.
Hopefully someone else has actual experience with this problem.
Hopefully someone can advise where to obtain the original Piston assy. part 24x199 with a 2 3/4" piston length. That would solve this issue. All of the replacement piston assy. seem to have only 1 3/4" piston.