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.
The other pins are probably still present internally or at least physically possible in that package, so only the pins in the positions where they actually exist external to the package are listed, this is not all that rare of an occurrence for packages.