Hi and welcome! I'm sorry but I'm not familiar with the B Class lineup. Have you studied the floorplans and specs on Winnebago's website? You might check out the Lichtsinn website and contact them for info; they are a dealership but very willing to answer questions (and try to sell you an RV!) Hopefully someone who is familiar with the ERA will see your post.
I don't believe there is any difference. people tend to use the "1" in front sometimes as Winnebago has it in places, but, there is just one 70M or 170M model; not two. I have a 70C (sometimes called a 170C) which came the year before the M and doesn't have murphy but has pullout couch. the slideout occasionally squeeks a little, but it's not bothersome. I could probably make it stop with some gasket spray.
I believe it has to do with the model year. Mine is called a 170x ... a 2010 model.
That's part of the issue. Winniebago had three models at one point: the 170x, the 170A, and the 70C... but the official part number for the 70C had a 170C in it.
Based on how well they marketed the ERA line, I think they let an intern handle the naming...
I'm new to the forum, so not that knowledgeable, but Winnebago's names and model numbers seem intended to confuse, not enlighten. They build a nice coach though.