i have five brooks saddles. four are the "professional" model and one is a recent vintage b-17.
i first bought a new one in 1982. it had hammered solid copper rivets. this was at the nadir of brooks popularity, i think, in that the saddle was on sale (denver) for five dollars. the shop owner said that no one wanted them anymore and he was basically giving it away. it has an "80" stamped on the back of portion of the chromed steel frame.
a few years ago i was refurbishing a few bicycles for personal use and needed saddles. i scoured CL and found three professionals. according the some brooks saddle type website, the pre 70's or 80's had a stamp on the underside of the leather that indicated date of manufacture, after 70 or thereabouts the stamp was on the frame. but no one seems to know how to decode the leather stamp. these three all had a stamp on the underside of the leather and small unhammered rivets.
my guess is that at some time all brooks professionals had unhammered small rivets. then, professional cyclists began hammering the rivets when the leather became overstretched. eventually, the public, seeing the the pros had hammered rivets and they did not, began clamerring for hammered rivets so as to be like the pros (not unheard of). and so brooks bowed to public pressure and we have hammered rivets. even the lowly (my favorite by the way) b-17 comes in a pre hammered version.
i'm just guessing on this...
feel free to write your own history and/or criticize mine.
huey