I think blackcurrant is the flavor used in MacBaren's "modern cavendish" that they put in everything.I'm having a bowl of this 1931 as I write, and before packing I really took the time to sit down and inspect the topping. I think I've finally figured it out. North Americans may not be too familiar but if any Eastern Europeaners can confirm this is a common flavor profile over there-- I think it's...........
Blackcurrant.
It wasn't easy to spot because there's a lot of natural tobacco aroma that mixes in, the topping isn't very heavy at all, it just adds a little special something to the smoke.