Ah, it appears the OP has stepped on what might be referred to as the "French landmine". My apologies for doubting your measurement, OP, it seems you could be right about the steerer being exactly 25mm in diameter. There's still a chance the bike is built to ISO standards, at least partially, but there's also the chance it's not. I know some of the old Peugeots did have standard ISO threaded BBs, for instance, but that's by no means all of them. Is this just a bare frame, or do you only need the headset to finish it off?