Hello, I took this Burris out of the safe tonight planning on mounting it on a 17hmr we got the nephew for Christmas. This thing is about 20 years old and its a 3-9x. It has an adjustment at the front of the eyepiece for focus but It's been so long since I used it I couldn't really remember what if anything this knob other was for. (see arrow in pic) Its right behind the magnification knob but in front of the on off for the red dot (see pic) and it spins freely and tightens and loosens but has no markings. Doesn't really seem to do anything just holding the scope up in the yard looking at the fence (but makes no sense that it would turn if it didn't have a purpose). I haven't used this in forever and can't find a manual online - If anyone knows or remembers if this does anything the help would be appreciated.
https://imgur.com/a/IYNPq8F

Is it the battery holder?

No idea.

It's not the diopter adjustment, that is at the rear lens.

And it's obviously not the magnification knob. Maybe it's there to produce resistance on the magnification knob?

I think it is a "lock" ring. Older Leupolds had the same type of ring. Although the eyepiece of your scope has the red dot/battery holder in the upright position -which is where it should be- I will bet that you can turn the entire eyepiece inward or outward by rotating it. That allows you to focus the crosshairs to your eyes. Once in place, twist the ring (marked with your arrow) against the eyepiece to hold it in place.

I'll take SRWAG (scientific really wild [censored] guess) at it...

Maybe the illuminated dot complicated the design of the eyepiece enough that the fast-focus diopter adjustment doesn't have enough range to do what needs to be done. So my guess is that the entire ocular assembly turns as a coarse adjustment and the knurled ring locks it into place and then the fast-focus diopter at the very end of the ocular assembly serves as a fine adjustment. If the entire ocular assembly DOES turn, then you'd want to be able to lock it in place so you will have the illumination switch on the very top.

I looked around on the web for a few minutes and couldn't find anything officially from Burris on this thing, and it seems that there were enough different "Electro Dot" product changes over time to make it even more obscure. I'm not even sure that what we're looking at is even really an Electro Dot offering...it may have been marketed as something else at the time.

Good luck figuring it out. Or better yet, maybe someone who has one and still uses it will see this thread and can tell you all about it.

What you are describing is how Night Force rolls.

A "jam nut" if you will.

Thanks for the thoughts guys. It may be a nut to keep the eyepiece from turning but i attempted it earlier and went at it pretty hard with hand strength and nothing broke free. Not curious enough to mess with it anymore since it had no effect on its future use. I guess ill tighten it up towards the eyepiece end and just leave it alone. Maybe someone will come along and know.

Lock Ring after having made focus adjustment