I have one of those on my Zieleman, probably the most comfortable brooks type saddle that I have.
Mine wasn't in as dried out a state as yours.
That said, I sent RHM a brooks frame and a really old B15 top that was found under the floor at
the brooks factory. I had him cut it and replace the top on the Swallow/frame I sent. The leather
was thick, old and dry as a bone. I contacted Brooks about it and they recommended Proofide only.
I know Tony C. (brooks repairs) has commented on same issue on CR list but I can't find it with the
new format (google groups).
Marty