If you have a venerable old FAS2240 or FAS255x, you can turn it into a disk shelf, by swapping the controllers with embedded IOM6 (IOM6E) for regular IOM3 or IOM6 controllers, and adding it as a shelf to a different controller.
The opposite however is not always true. It is not supported to turn a disk shelf into a FAS2240, but there are instances where it might be required or desirable, and I’ve seen people hit it a couple of times.
The FAS2240 PCM / IOM6E will work in any DS2246. However, while the DS4246 and DS4243 share an enclosure (the DS424), there are two revisions, and the FAS22xx/25xx only work in the newer one of them, which has better cross-midplane ventilation. Placing them in the older version results in a “FASXXX is not a supported platform” message and failure to boot.
The original version, X558A (430-00048) doesn’t support the embedded PCM/IOM6Es, while the X5560 (430-00061) does. If the shelf was shipped new after April 2012, it is probably the X5560. Some earlier may be also.