I cannot profess to know, but I would need more fingers if I were to count the number of times people have stated that ultrafine IS fp4 plus. The different dev times should not concern you, Look at the massive development chart where there are radically different times posted for the same films. I dont think you will go far wrong if you treat it as FP4 plus, ensuring you have a dev time which is curtailed for condenser enlargement. Lets face it, how many 125 speed films are there about: 2? and if it was plus X, surely there would be references to that effect, rather than FP4.
I could be wrong tho......