We had a thread on this code (triggered by a recent photograph on eBay)
here Chris and on page 289 of "Bomber Units of the Luftwaffe 1933-45 - Volume 2" [Classic, 2008] by de.Zeng & Stankey) the unit code 56+ is listed as belonging to the second formation of KG155:
Stab./KG155 (second formation) Nov'38 to May'39 and renamed Stab./KG55
I./KG155 (second formation) Apr'36 to Spring?'39 and renamed II./KG158
II./KG155 (second formation) Nov'39 to May'39 and renamed II./KG55
Cheers
Peter D Evans
LEMB Administrator