First of all, not everyone makes that claim. I know I have made that claim, a few other people have made a similar one, and many more have rejected it. And frankly I'm not sure I'm right; maybe I just had bad luck with two saddles.
Second, it is really hard to compare the shape of a new saddle with that of an old one, if the old one has been ridden. They change shape when ridden.
All that said, there have been a whole lot of models, sometimes different versions of the same model offered at the same time, sometimes different versions of the same model offered at different times, and I am pretty sure the same version of the same model can change over time. I have four (I think) B17's ranging in date from ca 1950, 1959, 1967, and 2007?, and they are all different; and I've seen photos of quite a few others that are different still. The widest range of variability I've seen was is in the B15 line. You can easily find more differences between two B15's, or two B17's, than between a B15 and a B17, depending of course on which ones you pick. Bottom line, the more I learn about Brooks saddles, the less I know.
Sorry I couldn't help!
I note on some of the professionals the leather on the nose is cut square, and exposes the adjustment plate. On others, the leather is cut round, and covers the adjusting plate. Are these different models, years or just what the builder felt like doing at the time? Also, some look much wider than others.