Locally we called them pig tails and cheater bars. Pig tails used a pig tail looking sort of loop on one side to allow some adjustability and flexibility to accomodate tank and band arrangements with different centers and to accommodate less than solid band arrangements wher some flexing may occur. They were I believe a pretty early design and I have seen US Divers models with flat chrome finishes that would date them from from the 50's or early 60's.
Cheater bars used a solid tube arrangement between the yokes and were designed to be used with tanks and bands with specified center to center measurements. Earlier ones used metal thumb screws on the yokes while the last ones I saw (sold by Dacor anda couple other companies in the mid to late eighties) used hex headed bolts for more security.
They worked ok as long as the tanks were properly aligned and secured properly in a decent set of bands (an awful lot of "ifs" involved) but they were by no means as secure as a regular doubles manifold.
Since they attached to a pair of regular K or J valves, they did allow you to shut off either tank and use a J valve reserve on one or both tanks. The ability to shut off each tank individually did protect you from a blown neck O-ring or burst disk failure but you still had only one reg and you had a total of 3 yoke connections to potentially fail.
They did save you the cost of buying another regulator which would have been required to go the independent doubles route. But that was the only "advantage" as they offerred no operational advantage, provided less redundancy and posed more potential failure points.
One of the big changes over time was that in the 50's, 60's and even early 70's all you needed was a tank with a J valve, a regulator and maybe a relatively inexpensive horse collar or even a surplus mae west to go diving. (no SPG, no stab jacket, no computer - maybe a cheap capillary depth gauge)
So the regulator was the spendiest piece of equipment and the cost of the regulator was a significant portion of the total equipment package. So using another regulator (on a pony bottle or a set of doubles) represented a very large investment and was not something most divers considered. Consequently single outlet manifolds of various types that would require only one regulator were the standard of the day even when single hose regs became common.
