I'm pondering similar issues while deciding on a Brooks. The original mattress saddle is falling apart on my '75 Raleigh Sprite and needs replacing.

The issues I have are that the mattress currently installed is on dual rails attached to a clamp to an old-fashioned seat post. A simple replacement of saddles would necessitate using the B66. However, I consider the Brooks saddle to be a long term investment, possibly outlasting this bike. So maybe I'd be better off buying a B67 and a single rail clamp to go on the current post. I'm also leaning towards the Aged edition, which only seems to come in the B67.

So pay attention to the rails. Though, I have seen ways of attaching dual rails to micro-adjust posts using a sandwich plate.