Sup military armorer dude (just guessing here lol).
Preban magazines are magazines created before the Assault Weapons Ban of 1994, which was a federal ban on many features we take for granted today. The federal ban sunset (expired) in 2004. Some states however implemented their own bans on the state level - NY, MA, CA, etc. NY specifically disalowed any magazines over 10 rounds produced after September 13, 1994. So basically there are a finite number of pre-ban magazines in existence - they aren't making any more of them. Getting ahold of a pre-ban magazine is the only way to legally avoid a Class D Felony in NY if you want to have more than 10 rounds.
Helpful link
There are three types of pre-ban magazines (basically).
- Undated - No date on the interior or exterior. Since the Federal AWB required manufacturers to put the mfg date on the outside, by default, any magazine without a date on the inside or outside is a preban mag. If it is an older looking mag without a date (and especially with a black follower) then it is probably a preban. The exception to this is D&H (off the top of my head) which doesn't date their mags - some unscrupulous sellers will take new D&H mags and put old mag floor plates on there and claim they are 'refinished' preban mags. From what I have read something like 90% of magazines that are preban are unmarked since it wasn't a requirement to date the mags and it was more of an internal QC thing.
[*]Internally Dated - see the link I posted above for pics, but basically the date is stamped on the inside of the mag body when you remove the floor plate. On OKay mags you'll see things like K4/92 (indicating it was manufactured in April of 1986) or sometimes just a 4 on one side and 92 on the other. These mags are more desireable because it is positive proof that the mag is a preban but a PITA because you have to remove the floor plate and explain the dating system to anyone questioning the mag's authenticity.
[*]Externally Dated - Magazines with the month/year stamped into the outside of the magazine. Most desireable because it is easiest to prove the magazine is preban. External dating wasn't implemented until a few years before the ban so these mags are much more rare than the other two types.
This all being said, post ban magazines aren't worth anything compared to pre-ban magazines unless it is a special brand (like Colt) or it has an improved follower (anti-tilt, self leveling, etc). AFAIK ban magazines aren't collectors items (and some people are needlessly paranoid about owning a magzine with "GOVT/LAW ENFORCEMENT USE ONLY" marked on the outside).