So Dave posited that with the hub in place, putting the large support wheel/ring on the top of the limber was an 'either/or' proposition.
I think he'es right. Though if I had moved the mount a bit more to the side... and built a slightly smaller hub (I based it off the Armortek hub... so it may be a bit larger than the original hub/drum)... Perhaps it might have fit with both the ring and the hub. Perhaps.
Here are some pictures of a quick experiment I did, just to see if the clearances might have worked. I don't think they would. But, again, some minor tweaks and... maybe. I will say that for those who don't want to make a wheel hub, a spare ring from Armortek would make an impressive addition to the Limber. Or leave it off the 25 PDR and mount on the limber with the Armorpax parts. Would make a very cool display!
This image shows that the ring will clear just fine around the spare hub. But the issue comes when you factor in the spokes....
Using some polystyrene I-beam sections, I mocked up a couple of spokes and, sure enough, not enough clearance. That said... a smaller diameter brake drum and a slight movement of the mount to the outside and.... maybe. Maybe it would clear.
Anyway, I won't be carrying the ring on the limber. It was more of an academic 'will-it-or-won't-it" question that I wanted to answer. My thought is... won't. But without trying an original... I'll leave it at.... maybe!
Thanks, everyone, for helping ID the mount purpose and for the inspiration to make the spare hub! It kind of adds a fun little element to the Limber.