There is no SM81B. However, there is the (discontinued) SM80, which is merely the SM81 body with an R104A omni capsule instead of the R104 cardioid capsule. The SM-81/R104A combo is the mike/capsule pairing I use most. It's very flat with good transient response and reasonable off-axis response, too. I have four of the R104A capsules, mainly so I can easily ensure that at least two are always up to spec.
Are those capsules available anywhere? I got a search alert email from ebay a few months ago for one, but it was gone before I got to the ebay website.
I've got three SM81s, because I saw the third one for cheap at a pawn shop. *Very* good for acoustic guitar. I usually use the XY pattern pointing at about the 20th fret.
Not if I can help it. They do show up on occasion, but not often. One weekend a few months ago, three came up for auction. I got two of them, which was very cool, especially since one of my original two is acting up and generating hum intermittently. I'll be sending it in soon to see if it can be repaired.