My guess was a B5N . They came stock on some road bikes in the 70's. Your finding them in the bin is similar to this post.They looked like the current b17 model but they were just tossed in a bin so I don't know what they were.
I called Wallbike today and he said the B5N was OEM on new bikes back in the 70's, its basically a B17N with a lower grade leather. He said that back then plastic seats were very popular and people would ditch the Brooks for the latest and greatest.
I remember when Brooks saddles were the cheapest replacement saddle you could get. I had them on my 2 bikes when I was a student because they were cheap. The local Western Auto Store had then piled in a bin without packaging for about $4.00 (the cost of a record album). I don't like them. They get wet and stretch. Even if you put a rain cover over them, water still splashes on the underside and they stretch and sag and the the sides wow out and cut your thighs. This happens even with proper conditioning. After the adjustment was all the way out I would turn it back and remove the rivets and re rivet it tight and start the stretch all over again. I also wrapped tape around the saddle to keep it from wowing out and cutting my thigh. I don't think they are worth $4.00.
I bought my B33 from Amazon and Brooks was none too happy when I needed warranty service. You're supposed to take it back to the place you bought it, which has to be an authorized dealer and they aren't.Who is the best place online to buy a Brooks? I did look at the other link for the imported ones, but want to compare to similar Brooks..