In the Season 2 episode of Rebels "Lost Droid" - we see Chopper obtain a replacement original strut to replace his mismatched (green) one
What model astromech droid is the green leg from?
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Chooper's mismatched leg could be from one of two astromech models including:
Chopper was a C1-series (C1 implying he was the first production model), so it is possible that this mismatched leg could be from another C1 series astromech, or another model in the C series, assuming that parts from future models are backward-compatible.
These were common astromech droids on Lothal, but were of inferior quality to other models of astromech. I'm including this model as parts from these models would have been common on Lothal, where Chooper (and the Ghost) were based for a significant amount of time while Chopper had his mismatch leg, so this could be a viable answer.
Please note that this is all speculation and guessing as it has not been revealed where Chopper's odd leg has come from.