AFAIK this switch only uses SWOS-lite, not SWOS … different firmware “products”, although versions might make you think these are related.
When the switch hit the market, it was shipped with a release candidate (v2.12rcX) of SWOS-lite, hence your thinking to go back to that version number.
This firmware had never been officially puplished, I think.
There are same threads in the routerboard or swos subforum, where other, older version (pre v2.13) popped up from time to time after users complaining, like here: http://forum.mikrotik.com/t/css610-8g-2s-in-no-link-on-sfp-with-intel-x520/146985/46
Edit: also, here: http://forum.mikrotik.com/t/css610-8g-2s-in-swos-release-tracker/146905/15
Note that the sibling (outdoor version, netPower Lite 7R) of the switch has a different firmware file, so look closely when searching.