Having had Germany Radar from it's inception, I have to say with some certainty that the CST (Coastal) sector was never a controllable sector in Germany Radar.
The Coastal sector, as it existed then, covered the very northern part of the Netherlands extending eastwards along the North Sea coast of Germany to the western coastline of Denmark and then going up to the boundary with the Copenhagen FIR.
These days, that same area is referred to as the JEVER sector with a base of FL245 rising to FL660. The sector is split vertically at FL365 into JEVER LOW and JEVER HIGH. The eastern half of that old CST airspace is now designated as the HOLSTEIN sector with the same split as JEVER.
The following links might help, although actual coordinates are not fully provided:
Maastricht Netherlands sectorisation
Eurocontrol Upper airspace chart for Germany, including sector boundaries
German and Maastricht Air route structure
MUAC Sectorisation chart
Good luck.
