The piston on the left came from the rear end of Kirby Palm's '83.  That's not dirt; that's all rust, including the line part way around the side on the sealing surface.  The OEM pistons are chrome-plated steel, and eventually the steel rusts and the chrome plating flakes off.  This one survived 17 years so one cannot fault their durability, but when they start to look like this they tend to cause pesky problems like sudden brake failure.

People who find similar conditions in their calipers often opt for stainless steel pistons so they don't have to worry about nasty surprises ever again.  The piston in the center of the picture came from Gran Turismo Jaguar (now defunct, and with good reason), but it's probably not much different from any other stainless steel piston.  Such pistons have their own problem.  The outer surface of the OEM piston was ground quite smooth, important since it slides back and forth on the seal in the caliper.  The stainless steel piston comes with an as-machined surface, pretty enough to look at but hard on the seal.

Palm's fix was to use a buffing wheel and buffing compound on the stainless steel pistons until they looked like the one on the right.  Takes a while, but hopefully results in a piston that will work well forever.

While dealing with the rear, Palm decided to tackle the front before problems occur there as well.  The front calipers have four pistons each, so a total of eight were ordered from GTJ:

The one at the front left is one that came out of the car.  Rust galore.  The faint lines on the sealing outer surface are not dirt; that's where the chrome plating has begun to crack and rust is starting to break through.

The other three are from GTJ.  As you can see, they don't appear to be a matched set.  The one at the back left appears to have about the same dimensions as the original steel piston.  The one at the front right has slightly thicker walls because the opening in the middle isn't as big.  And the one at the right rear has much thicker walls because the opening in the middle is much smaller.

Irritating, yes; this is the kind of product quality that GTJ became known for.  But this is a classic case of differences that don't make any difference.  If you turned these pistons over so the center hole was down on the table, you couldn't tell them apart.  The OD sealing surfaces are all correct, and the lips where the boots attach are correct.  They all worked just fine; some are a bit heavier than others, that's all.