I looked 5 other MS400s listed online, and all 5 had the typical "thin" swim platform.
After looking closely at the pic posted of the top side of yours, I notice the typical smooth gelcoat stripe that separates sections of non-skid just aft of the one large hatch cover. I am guessing, but believe your platform with enclosed storage/water compartment has possibly fabricated using the factory platform as the topic top piece and the balance (sides & bottom added to it. Those large hatch covers are roughly covering what would be the open "slits" in a factory platform.
The swim ladder also appears to be exactly as it sits in the factory platform, but in your case, a pocket to enclose it has been built around the ladder.
All very subjective guessin, but further inspection of the inside from the large hatches might confirm it if it makes any difference to you.