So it is! I must have the FC-M391-8 then, because it's definitely octalink.I stupidly assumed the '-8' referred to a part for 8-speed gear train and dismissed it, forgetting for a moment that the number of speeds at the back end has little bearing on the chainrings at the front.I'll double check the part number by looking at the actual crankset, but if I do have the FC-M391-8, then the BB-ES25 (126mm) seems to be the right part.