Questions about HP 85046B testset couplers 50Ω vs 7

Hi,
Can anyone tell me way the 75Ω couplers only go to 2GHz while the 50Ω couplers to to 3GHz? Does anyone know what HP did to convert the 50Ω units to 75Ω or are they completely redesigned?

Thanks

Swingbyte

After posting this on the HP forum, the answer is that in the old days of non-microprocessor VNAs it was hard to design and test good 75ohm standards so they only rated them to 2GHz.