Hello Michal !
The U.S. grid system was the same than the "British Modified System" in WW 2, here in this case obviously in the "Nord de Guerre Zone" grid. This zone covered Northern Europe as well as Tchechoslovakia. The grids squares were 100 km each and each one had a basic letter. Since the squares covered 5 x 5 100 km squares, about 5 "U" grids existing in the Nord de Guerrre Zone:
1- "U" North Sea and Duch Coast
2- "U" North of Berlin, Germany
3- "U" Eastern Frannce, Southern Border of Luxembourg
4- "U" Eastern Bavaria, a littel corner of Tchech Republic
5- "U" Middle of Slovakia
Which area is your desire now ?
You can look for the system in:
www.echodelta.net
Best regards,
Horst Weber