This product is either discontinued or we cannot get that from suppliers.
BRATECK LDT05-C012M Counterbalance Desk Mount, for 21.5 and 27in iMac, Easily Tilts and Swivel, Gas spring structure, Built-in spring tension gauge, Cable management, Aluminium, Steel, Plastic, BLACK
The iMac desk mount eliminates the need to remove the stand or modify the iMac in any way. The highly adjustable arm moves up, down, forward and back with just a touch.