Mostly, the B68 fits me and mounts on a single rail seat post. I rode narrower Brooks when I was younger and thinner, but they are no longer comfortable for me. Think of it as a wider B17 (which was my 1st Brooks saddle on my 1970 Schwinn Super Sport.
The B68 is a lot lighter than the sprung B67 (which I have on my Hubbuhubbuh and other bikes) and the triple sprung double rail B73 (which I have on my Guv'nor and is also out of production). The B73 is probably the most comfortable of all my current saddles. All of these are basically same shape as the B68. There are also several other double rail Brooks saddles of the same shape. I also had at one time, a triple rail Brooks B33, which was also very comfortable and weighed more than a complete bicycle frame.
I bought a bare B68 frame and had it chromed and then had Firth & Wilson put a Honey B67 leather top and large copper rivets on it for my custom - my Brook B678 Frankensaddle.
The B68 is also slightly lighter and much softer when new (especially the aged version) than the Rivet Loveland (but the Loveland does come with the chrome frame). It is also more comfortable than the slightly sprung B72 (I have one, not currently on a bike). If the day comes when I cannot find a B68, I will get a Rivet Loveland.
I still have the Brooks Professional that came on my 73 Paramount - still original, but definitely showing its age.
Laing