The number 250 on the nib is not a model number but a marking Montblanc used on steel nibs; quite why that particular figure was chosen has never been settled. Pens with nibs of this sort typically date from the war and the immediate post-war years, when gold was barely to be had for civilian production. On these pens the model number is not on the nib but usually appears as a three-digit figure on the end of the barrel. If it is missing there, the model has to be determined from the shape, the size, the cap trim and the filling system.
For telling the large pre-war and wartime models apart, the cap trim is a useful feature: according to collectors, only the 139 had silver-coloured rings, while the smaller 136 was finished differently. Then there is sheer size, the 139 being much the larger and heavier pen. A 139 with a steel nib is an interesting document of its time and nothing to take exception to, but collectors rate it below the same model with its original gold nib. When buying, bear in mind that steel nibs of those years frequently show traces of corrosion.