The square section seal serves two purposes. One to keep the fluid inside. And two, the square section distorts during brake application, when the pressure is removed the seal moves back into it's square shape and brings the piston with it thereby providing retraction.
If there are too many pits in the piston and they are in the wrong place, it's the retraction that suffers as the seal doesn't distort predictably. It is quite rare that the seal leaks because of piston pitting. But, if the piston pitting is severe enough, the brakes will drag due to incomplete retraction.
There is little to lose by trying the one you have. Unlikely to leak. But, if you start to notice brake drag after application, then shop for a replacement caliper piston.
As I understood it, the plastic washer served multiple purposes. As these were the first disc brakes, the softer plastic simulated the feel of drum brakes during initial application. Second, it helped damp out brake squeal. When new the nylon was probably a bit softer, and the original brake line rubber was a much harder, so swelling was minimal. Rubber fatigues when stretched. One of the reasons why SS brake lines are so appreciated, despite it's direct comparison to old fatigued rubber lines.
Cheers,
