After a fair amount of trial and error, it seems to me that the I/O modules are identical, and there is some key in the shelf hardware (backplane perhaps) that sets the personality to be either a D2600 or M6612.
As you've seen, a Proliant Smart Array won't even detect an M6612 in hpsum, presumambly because they are not a supported combination. What I have done, however, is move I/O modules from an M6612 shelf to a "genuine" D2600 shelf, and then I can detect and update firmware to the latest version (150). I have done this with many modules that originally had M6612 firmware versions on them.
I have successfully run the module (now) with D2600 firmware in the M6612, and have experienced no issues in a few months deployment. Hpsum still won't detect the shelf, but it runs fine in normal operation. I didn't test it beyond simple drive failures and rebuilds, power supply fails, and the SPP diagnostic routine, so something esoteric could always show up.
I'd prefer to use the "real" D2600, but most people selling them don't even know the difference. The M6612 just had an AJ832A sticker stuck over the AJ940A sticker. That's easily removed, and a it often is, since the D2600 is worth more on the secondary market than is an M6612. A serial number search (HPE warranty check) should reveal what it is, but the last one I looked up said it was a power supply. Oh well. I've also compared the Smart Array diagnostic report with different combinations of hardware, but many of the values vary, and I didn't really find characteristics that definitely established a particular "gold" combination.
