In all likelihood Z. While this is not an absolute foolproof method, try to enter the itinerary on ITA (with check availability unchecked) and see the fares brought up by the search engine for BD and UA and compare the fare basis under each carrier code for similar fares.

I strongly suspect that there will be a ZNC1S UA fare which is almost identical to BD's JNC1S.

If you have EF or KVS, an alternative method would be to compare fares for both carriers on your chosen route and look for matching fare and nearly matching fare basis name.

It is not absolutely foolproof in that there is no guarantee that the mapped bucket onto the op carrier necessarily matches that of the comparable op carrier's fares. But there is a very high probability that it will do.