I don't know about iMacs, but MBPros & MBAirs changed the SSD connector between 2012 & 2013. Besides, the only SSD adapter for the proprietary Apple SSD connector would be from Sintech....this is from reading this forum for years and this adapter is the only one that seems the most compatible & reliable. I'm not sure, but I think the 2012 models could only use SATA based SSDs. I think only the 2013+ Macs are able to utilize NVMe based SSDs, then only after having installed macOS 10.13+ (prior to installing an NVMe SSD).
FYI, if this iMac never had a PCIe blade style SSD installed internally, then it may not be possible to do so now. I know at least on later models that the SSD connector is not active on the Logic Board if the iMac did not ship with such as SSD from the factory.
As @rkaufmann87 has pointed out, check with OWC for possible upgrade options. They will know whether you can upgrade or install a blade style SSD into that iMac. Also, keep in mind OWC is the only vendor who tests their products are compatible with Macs and the only one who provides tech support for their SSDs on Macs. Not all SSDs are compatible with all computers.