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...I think it is a blue PX-10 from the 70's, that has had most, if not all of the components replaced with Campagnolo branded ones, except the rear derailleur, which is a newer than original Simplex updated one to fit the unique Simplex hanger. The Campagnolo decals and sticker were probably also added at the same time the components were changed out.

The 24"/60cm frames are hard to score in the US, so congratulations.