Steel rim questions

First off guys, this pertains to my '76 CB550 K and not my or anyone else's Kaw.
I'm looking to buy a new set of OEM rims (Star/Excel comes later) for my 1976 CB550K. The rear rim on my K is stamped 1.85x18. Well enough.
An ebay seller has a new, oem rim for sale that is stamped 1.85B x18. Can anyone explain to me what the "B" designates?
Of course I emailed the seller, asking the same question. His reply was "B stands for back, or rear".
I know he means well, but I also know that he is wrong.
Today at work I googled the rim size and came up with nothing, so I need one of you guys to fill me in.
Thanks,
Jim