Pretty confident that's each-way and not roundtrip as not seeing anything that low for roundtrip

Fare Basis code CSTR1000, 60-day advance purchase required, roundtrip booking required with Saturday night stay. DL has awards as low as 80K each way with a 90-day advance purchase (CSV800 awards). These would require booking in April and onwards to meet 90-day advance requirement. Awards in partner VS in Upperclass are available from 86K each way with a 60-day advance purchase requirement (CSMH860 awards). Neither CSV800 or CSMH860 awards have a roundtrip booking requirement. So you can do a one-way calendar search in each direction to find dates with availability and then combine in a roundtrip booking (to avoid ex-EU surcharges on one-way's). That won't work with finding CSTR1000 awards due to the roundtrip booking requirement (you'll never see availability for them with a one-way search).