You have a rev6A modded for sure, you can check the already mentioned jumpers JP2 and JP7A IIRC.
You can also use the megachip; but I'm not sure about the right way to upgrade the RAM past 2MB. You'll have a first 1MB chip thanks to the onboard 512KB + trapdoor A501 + mod, the second 1MB chip comes from the DKB. For more fast ram you'll need something like the Kipper's CPU RAM expansions (beware the sizes, 68000 and Agnus are close to each other!).
No bigger trapdoor RAM expansions seems to work with the DKB...
