The picture is very helpful. I can see why the knurled ring seems like something one would want to screw off, but from looking at the E2 manual, I think one just turns that to unlock and pull out the eyepiece tube to adjust the eye relief - it doesn't come off. Here's a part of the E2 manual on page 29:
View attachment 377415
Additionally, I've drawn a blue dashed curve to indicate the piece you're missing. It's an eyepiece (or eyepiece cover) that screws into the knurled ring. You can use the camera without it, but if you want a smooth surface that doesn't scratch your glasses, and protects the actual eyepiece from dust and damage, the cover glass is useful. It can be replaced with a corrective lens but you don't want that. The good news is, the E2 manual suggests that it is compatible with the same eyepieces as used by the F3 and many higher-spec DSLRs. (I infer this from the fact that it also uses the same DK-7 adapter as the F3 etc.)
You can verify this by measuring the inner threads on the knurled ring. If they measure about 22mm diameter, you're ready to go. Get a DK-17 eyepiece, $15 at B&H,
https://www.bhphotovideo.com/c/product/300678-REG/Nikon_4754_DK_17_Finder_Eyepiece_for.html or the generic equivalent, and it should solve the problem. The DK-17C is the version that corrects for different vision, but you don't want that.