As NavWeaps says...
Work began in 1937 as the main guns for a new class of cruisers limited by treaty to 8,000 tons. This project was halted in 1940 with the failure to produce an acceptable 8,000 ton design for this cruiser.
Canada had additional motivation to get a start on the concept even earlier;
I can't speak for anyone else's motives, but Canada's stems from how the RCN was formed; the RN dumped all the 5.5" white elephants on me, which resulted in the RCN adopting odder calibers than pretty much every other power. At some point, beuracratic beancounters decided that there was no need for another caliber between 4" and 7.5", and thus decreed that 5.5" wasn't all
that bigger than 5", and should work as a DP gun. Canada's now spent the last ten years or so trying to make that beuracratic musing a reality, with mixed results; the first attempts on the Argyll and Canada classes have no automation assists, and will suffer drastic problems with sustained ROFs if tested in combat. With those issues made appearant during training and excersizes, automation assisted mounts were researched, developed, and eventually placed on several classes with the triple-as-twin simmed solution. This was all in news reports for several years.
So, to reiterate, the automation in Canada's 5.5" mounts is intended for an acceptable
sustained ROF, not for a substantial increase over manual loading. The possibility for that in the future is there, but the technology isn't there yet. And, of course, none of this addresses how well they'll perform in actual combat; as I already stated, the pure manual guns on Argyll and Canada will have serious problems if they get caught in a sustained AA battle. If the Labradors and other ships with the early automation-assisted mounts had found themselves in combat, they'd have suffered teething problems (Now that they've been in service for 5+ years, I expect them to perform fairly well). I had a similar plan for the new-model lightweight mounts when they were introduced in 1940; If combat arose while the guns were still relatively new, they'd have teething and jamming problems for a while. I bumped the date up to 1937 as a reaction to seeing other players (without the 10-year development backstory) begin to fudge reports with much lighter weight 'automation assisted' mounts. :\
I can't really speak for the Russian, Bulgarian, and other such mounts; None of them have the extensive backstory I developed, and for the most part mate the technology with lighter shells that are less suseptible to crew fatigue, and thus presumably have different motivation.
In regards to ammo capacity, the new Tribal batch will have basically the same ammo capacity as the first batch, but has two additional guns to feed. That's something of a compromise given the fact that the hull is not being substantially enlarged to handle increased ammo capacity
and the new guns.