Have a look at this old Brooks catalogue from the V-CC library
1959 Brooks
It gives the dimensions and its B17 derivation as Rudi set out and was given the category for the "Sports Tourist"
The D refers to the fourth quarter of the year as the manufacturing date.

They were used by Raleigh (no surprise there!). Have a look through this very comprehensive catalogue for the saddles used. From the excellent threespeedhub online library.