Most likely someone put the late-model (90-up) badge on your '87. The trap (or foil-wrapped straight pipe that replaces it) is directly to the left of the valve cover, vertically above the turbo & exhaust manifold (as viewed standing in front of the engine). If you're looking at the battery you're too far away.
The engine number is stamped on the block, approximately above the injection pump mounting area, on the "L" shaped part behind the head. It's probably covered in grime though and VERY hard to clean without removing something like the intake manifold or fuel filter housing.
The 86-87 OM603.96x's had issues with the head cracking, possibly due to thermal overload induced by the trap oxidizer during extended city driving. The head was redesigned 3 times (4 heads) and the head gasket 5 times (6 gaskets). The 90-up OM603.97x, the 3.5L, had the "good" redesigned head & gasket but had bottom end problems with the rods & pistons. The 3.5's had a reputation of failing and requiring either a new engine or major bottom end rebuild, both of which would be in the $5-$12k range - PLUS labor. But the heads are fine on the 3.5L. They're great for donor heads, if you can find one, to replace a cracked head on a 3.0L. :p
Photos of my 603.960 head replacement are at the URL below. My car has the trap removed, so if your exhaust manifold looks like mine, you don't have the trap. Hint: The EGR valve accordion pipe connects to the end of the trap or replacement pipe:
http://www.meimann.com/images/mercedes/head_gasket/
Regards,