Piston question

[sxrchris]

Anyone know what this numbering means? One is 61x00 other is 61x02 can they be mismatched?

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[Yami-Rider]

Measure skirt with caliper or mic and see

 

[Zero Client]

Both those Pistons are seeing lean condition. The bottom one is very lean. Carbon buildup like that on the bottom is sign of lean burn/overheat. The crown getting too hot and then burning oil marks the stain. Too much and piston gets a hole.