I think that was the point of the design - to accept as many different plugs as possible. Remember, they come from a time when there was no NEMA, let alone standard plug patterns. I've even seen a variation that was not polarized.
I am not sure why you would need that or how you keep the 240v plugs out.
Yah, it makes me wonder why they are still making something like that.[with the tandem slots] What is the use? 240 plugs? New ones have a grounding prong. You'll either need a side cutter or a hammer to make that fit... I don't think I have seen an old 2 prong tandem plug for sale. I have seen some old 2 prong plugs with one blade tandem and the other straight, and it was rated 15 A 125 V. It wasn't terribly old either.I wonder if this receptacle is dual rated like the old ones were, something like 15 A 125 V and 10 A 250 V. Note: Non UL. it says. Perhaps it is manufactured for other countries such as Mexico or some South American countries that do not have listing requirements?
I seen this pic posted on another board, and thought (Or hoped) someone photoshopped it...
Interesting how the slots are not centered (vertically) on each outlet, but are in the same position as they would be for a grounding-type receptacle. I presume that the insides are the same as a 5-15/5-20/6-15/6-20 but they just use a different front.