What is considered "standard" for the 2 pole connectors on semi tractor/trailers. 

Vertical? Seems the Shur-Co tarps we have mostly have the vertical oriented 2 pole connectors. But then, we have one trailer with a horizontal 2 pole. It came that way, bought used.

Then after I ordered a couple of 2 pole receptacles to put on the trailer, I discovered that some of them have pins that are further apart than others. The "narrow" plugs won't fit the "wide" receptacle. Looking at online products, I don't see any mention of the wide or narrow pin orientation. The Shur-Co branded plugs/receptacle that we have are "narrow".

I'm considering spending a day & making everything the same. But, what is considered standard in the trucking industry? These are the plugs that must be able to handle about 50 amps and about 6 ga wire for electric tarps, etc.