I took the heads off today and measured the stroke and they both seem to have the same travel unless one piston is shorter than the other which I won't know untill I strip it right down. There seems to be different markings on the pistons heads as well as a concave dip in one of the pistons.

Piston markings are
2100x - STD 16 Flat piston
2105x - STD 15 (40404) concave piston

Any info or history would be great. Thanks

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I've got a highway to mow