I reckon you're spot on codhead - there's no substitute for high-quality design, it's timeless.
I dismantled the Duel again tonight to have a more detailed look at the gearbox before it goes back to the owner.
If anything, it served to reinforce my respect for this design!
Here's what I found...
Here's the gearbox disassembled.
Pull out the right-hand 'idler' gear.
There's a brass washer that sits atop this gear, but usually sticks to the reel body when it separates.

Pull out the two-speed assembly (which incorporates the anti-reverse bearings).
Slide off the two gear cogs to reveal the dual anti-reverse assemblies.

The six rollers slip right out, but you'll have to remove the push-rods from each cavity.
The springs simply slide in.

I've cleaned the gear shaft well as, like all anti-reverse bearings, grease diminishes its functional efficiency.
Slip the push-rods back into each cleaned cavity and realign the rollers.

Here's the first gear and anti-reverse mechanism fully assembled.
For interest's sake, here's the separated spring and push-rod.

The spring slides down inside as mentioned earlier.
In order to get the gear on over the anti-reverse rollers, simply place the gear over the assembly, then slide each roller in afterwards.

Here's the finished two-speed and anti-reverse assembly, and inserted back into the gearbox housing.

Replace the second 'idler' gear and brass washer, and... job done.

Reattach as per the original post above.
Given the wet sump protects the gearbox from build-up and corrosion, I reckon this design this basically flawless.
It's a beautiful bit of gear and was a pleasure to work on.
Righto - all the best, Justin