Ahh, itâs called top assembly, so it didnât click for me until I just tried it.
I see now, 300mm between the outer points of the locking pin slot, plus every other measurement I need.
Thank you, everyone.
Most regrettably, the strength of the âlockâ is insufficient; the ball just moves too freely in the socket no matter how hard I tighten it. Canât help but wonder if a TPU insert inside the socket would work better. HmmâŚAnyways, Iâm fairly convinced that a hirth joint is my path forward (see picture). It locks securely, can withstand much greater forces than the RAM ball-and-socket solution I first explored, and will satisfy my need for custom positioning of accessories. Back to the drawing boardâŚ
Canât be a version two, without a version one.
I think that hirth joint is a good idea, really accentuate those ridges, even angle them slightly on each side so theyâll pull together when screwed, better for 3d printing as well.
Two of these together and the inset nut and thumbscrew and youâre set.
This is very similar to the style I have for the aac mount it locks into place and can be adjusted I will also be using this for the tray table to allow it to be tilted as needed. I find that it does work really well 
I printed the âscrewsâ with .20 layer height with a .6 nozzle and they worked fine.
this is great! can we get a prototype model to play with?