You already have a natural grooved cut line. If you don’t have lathe access you can do this yourself with a crisp hack saw. Just go slow, and follow the groove. Vice it up with a piece of leather to protect it unless you have nylon jaws or akin to it. Keep it nice and jawed tight, without it being held at a vertical slant. The more it’s close to 90 degrees in the jaws the better your cut will be. Again clamp it vertically so that you don’t even to begin crush it. Then it’s definitely toast and essentially non-threadable. You can always rotate it in the jaws too if that helps you stay within the groove. You do have a clear view of how your cut’s going by looking at it constantly from the side or eye level.

(In fact, doing this on a lathe has its inherent risks because of inadvertantly applying too much jaw pressure and crushing it - making it even minutely ‘out of round’ thread-wise is toastville. Milling it off vertically in half or so increments with a mill is kinda safer but you prolly don’t have access to one either I assume.)

Then take a sheet of 200 grit metal sandpaper, or medium grit, put it on top of something FLAT like a sheet of glass. Take your now parted off lower piece and rub it around in circles paying attention not to tilt it. Get it as even as you can, then finish it off with fine or ultra fine grit same sandpaper. You can dampen it with water of course too when doing the final sanding.

I would then polish off the anodizing (or use something like Greased Lightning - available at Home Depot, etc.) , if you don’t want to cover up the now exposed silver aluminum cut with a black marker.

Then just buff it up to a shine with auto type mag wheel polish, or take that ultra fine sandpaper and gently abrade it to give you a dull satin type finish.

I wouldn’t grind it down either. Believe it or not that’s a lot of manual grinding much less to keep it half way even. Also too easy for most home hobbyists to add another crenallation or at minimum a flying hot bezel. :laughing:

PS. I can’t tell from the pics if that groove cut off still retains your reflector. If it doesn’t scratch all the previous but at least it gives you an idea of another way to part things up manually.