I took a closer look at the model number and its def EJN-75 and looking closer I can see where there was supposed to be an additional 7 but it didn't get stamped all the way, just the top part.
So it is a EJN-757
barrel does look micro groove and I remember when I originally picked it up in the store I initially thought the barrel was shot out.
Based off it's styling and other's I've seen I always kind of guessed it was from the 50s. It also came with a 2 piece scope bases, and a hawthorne 4x scope that had a rail along the bottom of the scope that the bases clamped onto vs the more common rings I'm used to.
I actually had to kind of hybridize the mounts I'm using now. I had some Warne 2 piece mauser bases but the screw holes didn't line up. So I tried a Leupold 1 piece base and the screw holes lined up, perfect... then the screws didn't fit. Just to try it I figured out the Warne screws fit.... So it has a Leupold base with Warne screws.