I have used Brooks saddles for many years, and the ones with holes in the base on either side of the leather have been an option, but not for me, although I think I do have one on either on one of my bikes or Machka's.
They are intended to compensate, I think, for extensive weight on the saddle which pushes the top down and therefore the bottoms out so they start to irritate the inside of the thighs and they pass of the edges. Adjustment of the end-of-saddle bolt system can compensate for some of the slacking.
I think a thin but strong cord (I think maybe fishing line as in heavy duty stuff) can be used to tie the sides together underneath so they can be brought back from irritating the thighs (or wearing out pants/shorts). The cord does need to be threaded so that any ties are on the inside the saddle.