Mystery of the ДPУ

Might have finally found out what the У(U) stands for in russian tubes that also have the ДP(DR) designation such as the 6Ж49П-ДPУ  and 6Э6П-ДPУ. 

IIRC the max frequency of the standard tubes is 200MHz, but I found a listing from a Ukranian that mentioned the U has the meaning "continuous action gold pins and maximum operating frequency of 600MHz". 

JC's initial guess that these may be suited for UHF seems on the money. As I can't find any kind of datasheet for a DRU type, only DR, it is hard to pin down.