They are all overclockable, but only with the reference cards can you safely unlock the 6970 shaders. This is because the reference cards have a dual bios with a little switch which
afaik the non-reference cards don't. I think for most models you can unlock them but without the dual bios if the flash goes wrong you can brick the card and have an expensive paperweight. There is no danger with the reference cards because you can simply switch to the other bios if things go wrong.
If you can't see the AMD logo, then the reference cards can be spotted because they all have the same stock cooler
.