I have it sent off to a watchmaker to get it serviced and get the serial number off the movement
Here is another 20B. Even though I'm pretty sure that the case was not made by Longines, there are matching numbers case/movement (18007).
Same with some other private brand Longines. I assume that most 20B and 16D were shipped to the US without case, so I wouldn't say that it is not original.
Are you sure that isn't a Longines made case? It sure looks like one made in the style of a Longines case. Have you asked Longines? I don't know the answer, but it certainly looks like one of the case suppliers used by Longines and the engraving style is very much same as Longines. Do you see the "Illumination" of the capital letters is an empty outline on top half of the letter. The bottom half of the letter is filled in. There is a heart shaped figure that divides the top half of the letter and the bottom half of the letters. And the fancy swirls around the letters. Does it not resemble the other AA4 case engravings? Not to forget the surounding metal file looking diamond patterns. There would have been only a couple of people in the factory doing this type of engraving so there is a "style" to it that helps identify that it most likely was the same Longines engraver that did this.