LAST POW BLOCK is 21600
OP was incorrect with 20160 that was referencing the last half.
https://github.com/SonicScrew/SonicScrewdriver/blob/master/src/main.cpp#L972
https://github.com/SonicScrew/SonicScrewdriver/blob/master/src/main.h#L48

As you can see here PoW cut off was 21600. OP was updated to reflect proper amount