If you mean the move before the spinning rear elbow while in seiunchin stance, I see the combo as an elbow break. IMO, armbar alludes to a submission hold requiring maintaining it for a while. Not the karate way. Not a throw and probably not even a takedown (as there doesn't seem to be hip or leg leverage for that) but capable of swinging him around.
Yes, that's what I am referring to. Elbow break when hyperextended over the shoulder. I referred to it as an armbar only because of the commonality between the two moves - opponent's thumb must be up to properly cause the hyperextension over the shoulder of their elbow.
However, by pulling the opponent closer to you as you pull their attacking hand over your shoulder, it can be developed into a type of Judo-esqe ippon seoi nage, if you will. Loose and sloppy, perhaps, but still a shoulder throw. In this case, the rearward higi no ato tsuki becomes a helper as you bend forward to complete the throw.
Again, not obvious or commonly-taught Omote (it is what it appears to be) bunkai. More Oyo.
In my dojo, we talk (laugh) about the 'secret scrolls of kumite', which begs the question - if they're secret, how is it we know them? But more clearly, these are taught in our dojo at the advanced level, and I'm a mere beginner in that realm. My sensei learned as a direct student of Harrill and Mitchum, who between them spent a lot of time developing bunkai and passing on received training. We've also watched and practiced the videos made by John Bartusevics, a direct student of Shimabuku.
I've seen dozens of techniques developed out of things as simple as the final move in Wansu (one hand kamei) and the stand up after the final kneeling punch in Chinto. It's head-spinning with the sheer number of esoteric and potentially viable applications. My shins seem permanently ouchy-d from being uki to the step over in the first stepping move in Naihanchi. It goes on and on, endlessly fascinated.
I've gone way off topic, but I wanted to add that according to my sensei, both Harrill and Mitchum were also huge fans of the basics. Block 'em and clock 'em with a middle body block / straight punch. Works really well. My sensei told me that both men could deliver a side block that would cause your soul to leave your body.