I have two magazines with Colt floorplates and, with the exception of one small mark on one mag body, no other marks anywhere.  This small mark looks like a I inside a U, located on the rear of the mag body, close to the bottom.  The followers are cast metal.  These  were advertised as used Colt 20rd magazines.  They seem to function well and are in very good condition with the only damage being the finish.  The reason I ask about how to identify Colt magazines is because the finish on the magazine bodies is well worn but the floorplates look like new.  

The "U-I" symbol is for Universal Industries who did indeed make the mags for Colt.



The alloy followers were original and NONE work better!



The finish on the early twenties seems to come off when you look at them!  Your floorplates should be marked either with a ".223" or "5.56"- the .223's are earlier.  For more you can check the mag FAQ here:

[url]http://magfaq.tripod.com/index.htm[/url]

IMHO- the AR was designed with the twenty rounder in mind.  It is THE most reliable mag- period!

HTH- GoGo

If you look on the mag with the UI on the side it becomes a CH for colt hartford

"the magazine bodies is well worn but the floorplates look like new."

what exactly is stamped on the floor plate ?

The floorplate has the Colt symbol with a circle around it in the top left corner.  This is followed by;

        Cal 5.56MM
 Colt Firearms Division
      Colt Industries
  Hartford, Conn. U.S.A.

just a hunch...!!!

what you have there are old "Colt AR-15 .223 cal." mags. that someone took the floor plates off & put them on the newer bodied mags & sold them at a higher price, then put the "5.56 cal" FPs on the "well worn" bodies & sold them to you...,

just a hunch !!

as to the "U I", what GoGo & ekg98 said is true, & that symbol dates back to the first produced Colt 20 rounders, i am not sure it is used any more, unless they are going to the military.