You can either send your slide off to be milled, or buy a slide already cut, new slide would need fitted to your frame, a barrel and bushing fitted. Would be best to send your slide to a reputable 1911 gunsmith who has such tooling
If you plan to check Caspian, wait for a couple of weeks. At the moment, most of the front office is in Las Vegas at the annual SHOT Show, so there's nobody back home to answer such questions. The show runs through Friday, but give them a few days to get home and get back in the saddle.
The point I'd like to offer you is that you don't have to ship your gun.The frame is the serial numbered component..."The gun"
The slide is a part. You can just send the slide.
If your Colt is a Series 70 yes it can be milled for a low mount reflex sight such as a Doctor sight.
If you Colt is a Series 80 no you can't do the low mount cut because of the Series 80 parts interference.
Yes I did machine the sight cut and yes the slide is drilled and tapped so the sight can be mounted directly to the slide.
When I was in the business I did a lot of low mount sight applications, there's several pictured on my website.
The sight is a Doctor sight which in my opinion was the best made at the time and still may be.
Here's a better picture of a low mount reflex sight application.
Don't let the ears in the back fool you on the depth of the cut as they've been cut down to clear the adjustment screws of the sight.
The milled sight cut is 0.250 deep.
Yep, actually more then 0.250 can be milled from the 1911 slide but I don't.
The cut I made sets the reflex sight plenty low enough on the slide plus leaves plenty of slide to drill and tap for the two mounting screws of the sight.
Milling the pocket for the sight in the slide reduces the fore and aft movement of the sight reducing stress on the sight mounting screws.
Thank you for the compliment on the guns I've built.
Yes they were smooth and accurate.
The first gun I pictured with the long slide and Tribrid barrel shot groups so small it would make a bullseye shooter envious.