The cover glass is welded to the LCD and not replaceable on its own. Sorry ;-{
'''For reference:'''
[https://www.thebookyard.com/product.php?cPath=34_166_374&products_id=15277|iMac 27" Retina 5K LCD Panel & Front Glass Assembly (14/M15) - New] Apple P/N 661-00200 - Either a LM270QQ1 (SD)(A1) or LM270QQ1 (SD)(A2) LCD unit
[https://www.thebookyard.com/product.php?cPath=34_166_459&products_id=16391|iMac 27" Retina 5K LCD Panel & Front Glass Assembly (L15) - New] Apple P/N 661-03255 - LM270QQ1 (SD)(B1) Which has for a LDC unit.
You’ll need the second display for the correct LCD panel. Here is the Grade B version which is much cheaper! [https://www.thebookyard.com/product.php?cPath=34_166_459&products_id=16467|iMac 27" Retina 5K LCD Panel & Front Glass Assembly (L15) - Grade-B]