Should my new 203mm adapter go over the existing one? Do they stack? This is my fork: 



and this is the adapter I got (SM-MA-F203P/PM):


(click to enlarge images)

The adapter is missing screw holes on the sides to fit on the fork. The bolts it came with are long enough to fit through the brake calipers, the 203mm adapter, and the adapter currently on the bike.

Does this mean they're meant to stack? Or should I go and buy another adapter with holes on the sides so I can remove the existing adapter and only fit the new one directly to the fork?
You bought a post mount adapter but needed an IS. Stacking it should put the caliper in the right place and would likely more or less work, but it's not a good idea given the alternative of getting the right part.