A replacement MB block or engine won't have a number by the IP. It should have a tag at the aft end of the block on the turbo side below the block heater. IIRC the 7th and 8th digits tell you if it was a replacement short block (no head), long block or complete engine. Here's a sample tag from gsxr's collection:
I don't know how Metric and other rebuilders identify reworked 603s.
If you don't mind the work of replacing the engine, don't dismiss an iffy 3.5. You can swap in a 3.0 block for little more than the cost of a good short block and a head gasket. Under $1000 bare bones to $2500 if you succumb to
while you're in there's. Don't make a decision without pulling the head to measure piston protrusion to confirm bent rods. A bad head gasket oil seal will cause symptoms of a bent #1 rod.
Sixto
87 300D