Are you trying to book the Delta numbered flight or the Code share flight ?
The code share flight may begin with 4 digits of which maybe the last two
match the Delta flight i.e. ATL-AMS DL 0072 and KLM 6072.
On the TravelNet site the KLM 6072 is greyed out and cannot be listed for
but the DL 0072 can be.
For flights under the KLM schedule the work-around is to purchase a ZED fare.
Can’t answer questions any further all my information is dated.
Suggest you try another more active forum LINK: http://www.nonrev.net/delta-air-lines