The double post plugs would allow the running of a twin spark head with a single (large) coil from cylinder.
A large quantity of those First American plugs was acquired by one of our fellow spark plug collectors several years ago. They are fairly plentiful. Not sure how a set would run in a car as the "electrode" being a metal plate would seem difficult to adjust a gap on.
The "series plug" was available in two types. The one shown in your post above (from ebay) is made to connect two plugs (in series) to run in one cylinder. There is another style but with two separate ground lug and two separate electrodes in them. This is a "dual" plug designed to connect to two ignition sources (both battery and magneto). Such a setup would produce a spark from either of two different sources. It's also possible to produce two sparks at the same time, if one could time ignition from the different sources so it would occur both together. Amazing things they came up with back then. Here is another oddity - a double ended plug. When one end fouled, just remove the cap, turn it around and insert the clean end into the cylinder. Note the cap has a clear mica window so you could watch it spark, thus ensuring it was properly working.
I'm hoping this winter to find a person to come here every day and put stuff on ebay and Craigslist for me. I have tons of stuff including about 100 spark plugs I got at the Shugard auction in Spearfish a few years ago. I bought a bunch of them for big bucks, let Steve Jelf and Neil Chichoine have a bunch, then bought everything left. Never looked at them again after I got them home. I think there are some of those First American in there and some other odd ones. Let's see, 100 plugs, $50 average = $5000. Hmmmmmmmmmmmmm. More research needed.
Careful with the math Stan, don't want you to short-change yourself on the plugs. Really, there are 50 cent plugs and 500 dollar plugs these days, sometimes even more depending on the plug. I've been collecting for over 40 years and would be glad to give you more info on them anytime. Drop me a note with a list of some of the plug brand names when you find one of those winter evenings with nothing else going on. Or, start your own collection.