Early and late rods?

Pulled these out of the motor im rebuilding. 1 piston (one on left) is a different casting to the other 3 and is physically fatter down the rod.
Why would this been done??
Someones gone to a little bit of effort cleaning up the castings but I don't know why theres 3 of the std looking ones and 1 of another sort I have seen before.
Why would this been done??
Someones gone to a little bit of effort cleaning up the castings but I don't know why theres 3 of the std looking ones and 1 of another sort I have seen before.