Does anyone know why some of the Great Lakes Aviation flights have UA flight numbers and some don't? For example, AMA-DEN is operated by ZK and has a UA flight number, but WDG-DEN, operated by ZK, has no UA flight number.

Furthermore, is it possible to issue a UA award ticket on a flight operated by ZK with a UA flight number? What about a ZK flight without a UA flight number? (my guess is yes to the first and no to the second)

I know that you can issue a UA award ticket on US Airways flights that have no UA flight number, but that is because US is in the Star Alliance. I don't see ZK in the Star Alliance, so I'm guessing that ZK-only flights are not available for an award ticket or earning UA miles.