What I did hear is that after the first batch in November December 2012 the one that come after all had the spare sata port also if not purchased with the fusion
So unfortunately, the only way I'll probably know for sure if to open up mine (27" btw) and find out whether it actually has an extra SATA port or not. 🙁
So unfortunately, the only way I'll probably know for sure if to open up mine (27" btw) and find out whether it actually has an extra SATA port or not. 🙁
Avoid OWC they are severely overpriced. Your best bet is an official Apple blade SSD from a retina Macbook pro or iMac. They have TRIM enabled by default and obviously are fully supported hardware wise. eBay/Craigslist is your friend.
There is one tiny caveat to your question - you definitely have the slot but the retaining screw for that slot to hold the blade in place might not be present.... if it's not present then it's easy enough to get but worth thinking about.
This guy has a base 27" with the regular 1TB HD and his logic board has the blade SSD port and the screw hole but no screw, which would need to be obtained separately. As All Taken has said, it is pretty apparent that all 2012 27" iMacs are equipped with this port.