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In the Kowa HighLander binoculars series, there is a black ring at the top side of the eyepieces... then there is a silver or light-colored ring (or, area) below that on both the lenses... and then there's another black ring or area below that. Where in this sequence (black, light-colored/silver, black, then the color of the main body of the binocular) did the separation of lens and binoc. occur? i.e. what of this set of rings or areas, came away?
I suspect that the clamp or other securing / tightening mechanism that held the lens in place, has slipped so that it is now too tight to re-insert the lens/lens assembly into the binocs. So you might want to find the thumbscrew or other tightening control/knob-let, and loosen it a few turns until the lens / lens assembly fits back into the binoc.
If you wish to send two pics (or two in one pic) of the separated lens/lens assly., and the place, looking into the binocs, where the lens/lens assly. went in / came out of, that would help me advise you about this problem and what best to do about it.
Excellent. The mounting for the gray part, please note, is screw-threaded inside, but not outside. There is, however, a tab protruding from the outer case plastic... that, presumably, allows the gray part to 'lock on' to some extent, though I cannot see the inside of the gray part to make sure of that.
So my idea is, for you to look on the inside of the gray part that came off, and see if there is a slot for that protruding tab, and see if you can put it back on so that the tab locks the gray part on. Presumably you twist it into place slightly.
If you need any other guidance, please let me know.
How much force did you use to get it off?
Were you unscrewing the eyepiece at the time, and did the eyepiece seem to be difficult to get off? or, how did it go?
What does the inside of the gray part, that is to go on over the tab, look like?
I don't think I can advise you without knowing that.
I suspect that, since this is a new binocular, you simply have a defective binocular. Badly assembled, or otherwise defective. Returning it to the vendor or store as such, as defective, would be only appropriate. (Given that you removed the gray part with minor traction, that could be the case.)