E39 mogul base-What does this mean?


Attempting to replace my Hamilton 175w MH bulbs with an other brand.
This brand states that it fits E 39 mogul base. Is this a standard size?
I've read that their is an E39 European and an E40 US and they are ususally interchangable.
Does anyone know if this is true? I just want to be sure before I order them.