Why didn't the pin numbers follows sequential numbering like 1-2-3-4-5-6.
I think 2 & 5 are taken as common center-taps,but OEP1200 have only 4 external pins.

 

My guess is that the part is available in a center tapped primary and center tapped secondary and it was easier for the manufacturer to use the same package for both models. It was easier just to preserved the pin out assignments across the two similar packaged products.

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