The a/b crap came from howie and is an incorrect interpretation of the same stamp.
Actually the a/b started with the survey, so I'm the guilty party for that one. I just needed to call it *something* and we hadn't come up with a ? designator yet. Howie calls the mark straight 0306 or 0406 depending on which part of the left leg appears to be closed. This mark is considered /0406\a:
though to be honest I've never seen one in the wild.
After looking at all of the images I have of them, I think that the ?14 image that LC shows in the arsenal stamp FAQ is likely the correct one for all these stamps, and the ones without the legs are grouped in a way that a broken stamp is the most likely culprit (they probably only had a single stamp for a long period of time as this factory's production was only ~30k / year max.)
What everyone chooses to call it is really up to them I think. Going forward, SKS-Files stickies, FAQs, and the new updated survey will likely call it out with a ? number and maybe we'll make reference to people referring to it as '0306'/'0406' in the past so any older posts on other boards can be more easily web searched. At some point in time, hopefully we figure out the correct name and go with that from then on.
We'll need to research this factory a bit more and see if we can tease out any more tidbits of data. The threaded to pinned transition is extremely relevant, this is the only factory I know of where this occurred (though you have to figure there must be others). Also relevant is the 11th year production of the first gun in the list, 1966 means these guns were well out there for use in VN. I remember reading about a supposed bringback with this stamp on it long long ago, but never quite really believed it until now.