Although the original message specified Colt magazines, it's probably worth pointing out that other 20-round straight magazines of the same design are available with different brand names. For example:
RGUNS.net . . . and. . .
Eagle FirearmsThey don't have Colt stamped on the bottom, but they are made to the same specs. You could use these and save your OEM Colt magazines as collectibles, in case you ever resell the rifle.
Also, sometimes used or surplus 20s come up on Equipment Exchange. Last week I got a box of USGI Adventure Line mags that look like they've been in a time capsule since 1970.
As far as I know, the new curved 20s are perfectly functional. They're slightly longer and they aren't allowed in service rifle competition (because they aren't mil-spec). They were invented to make life easier for the manufacturer, because they can be made on the same tooling and accept the same followers as 30-rd mags.