My iMac 21.5 uses a 12.7mm optical drive. I was able to use a 9.5mm optical bay adapter for an SSD. However, there was a slight difference in screw placement. It worked but I had to angle the screws slightly. As I recall the screws were slightly closer to the overhang of the optical drive.
Then again it was some cheap no name knockoff of the Optibay I bought on eBay. It could have just be lazy manufacturing.