EASports, I've been doing some research and asking questions about those receptacles you posted....as mentioned by Lostazhell, the second receptacle (t-slot) was the successor of the first one.
From what I have gathered so far according to some on this web board, sometime back before 1930 the electrical industry had not yet standardized the plugs on different appliances, fans, lamps, etc.....therefore there were several types of male plugs, however the two most common were the parallel blade and the tandem blades.
Then finally the electrical industry decided all 120 v. plus will be parallel blades and not tandem blades.....but yet many tandem blade plugs still existed, so they invented the t-slot to accomodate both parallel and tandem type plug ends.
Of course what makes me curious is what was wrong with the first design arrangement of the slots that they felt they needed to change them over to the t-slot design?
The first outlet you have posted will most likely accept a standard ungrounded 120 v. plug. Looks like if you were to take one of those 15 a./240 v. NEMA 6-15 plugs and cut off the ground it would fit in there too.
Of interest when I first started this research I asked a guy who had been selling electrical stuff since the 60's....he told me the tandem ungrounded receptacles like these could be wired for 120 v. or 240 v., the 240 v. using the tandem blades, but others here have mentioned that is not so. Of course this same guy said because NEMA had not standardized anything until the 60's that it was very likely to plug the wrong appliance in...
Now on that 2nd outlet....what does that cover say? Does that say "PAT 10-20-17"?
How high on the walls are these receptacles located? Perhaps they're still live on a 120 v. source?