FWIW on LH B class maps to Economy Flex when flying SK. Looking at a OS coded SK flight such as SK7682 the fare structure is CDJYBMHQ...

So the question is of course if B happens to be on the right side of the curtain? I have flewn quite a few times on OS B fares but then only in OS metal.

Did you get a seat assigned? That may indicate if it is Flex or P-class.

Let us know the result. Always intersting to know for future trips.