Brooks first made slotted saddles about one century ago, and has just recently succumbed to market pressure to reintroduce them--or it, in this case, since there's only one (but it comes in a mens and a womens). 

It is called the Imperial, and is virtually a B.17 with a slot on top and lace-holes along the bottom rim. Several olden-times saddles  had lace holes, but they aren't entirely a nostalgic throw-back: When you remove leather for the slot, some structure is lost in the process, and the saddle tends to go swayback early on.
Lacing, which amounts to tensioning the saddle, perks it up quick, and Brooks even supplies laces, in four different colors of them---black, red, turquoise, grey. Thank goodness it doesn't suggest you can change them "to fit your mood or personality!" 

You want to know how the Imperial compares to a normal B.17. Accounting for the normal fantastic amount of subjectivity that is unavoidable in any saddle review, here's one idiot's opinion: