The screen is glued to the glass. I have not heard of anyone separating them yet. It's Apple part #661-7169. Can you give us a link to the part you're looking at?

Here's the iFixit teams review of the 2012 21", it's the same for the 27":

The new iMac, with its ultra slim bezel proved to be a disappointment to the iFixit team, because both the glass and the LCD are glued to the iMac frame with strong adhesive, which negatively impacts repairability.

The late 2012 iMac 21.5" – code-named EMC 2544 – is an exercise in disappointment for us. We were quite worried when we saw that super-thin bezel during Apple’s keynote, and unfortunately we were correct: the glass and LCD are now glued to the iMac’s frame with incredibly strong adhesive. Gone are the lovely magnets that held the glass in place in iMacs of yesteryear.