W140 S350 strange R603 block number.

[Hardus1]

So I have a 1995 W140 S350 with the 3.5L OM603, I was trying to find the block number for cross references some parts for a rebuild. I've searched and read a few post and it showed that the block number was suppose to be near the injector pump but could not find it, however I eventually, "assuming", found the block number as it was located near the passenger side bottom of the block and it starts R then the usual number but it looks like this R 603 011 35 01 https://showbox.tools/ (see pic).

I didn't find the Original Mercedes-benz Tauschaggregat metal tags anywhere and the top of the pistons says 96/6 (see pic) so I'm assuming that it may be an original crank rods and blocks. I've look around at other OM603 blocks (both 3.0L and 3.5L) and I didn't find any that carries the letter R in front of the block number.

Anyone can give some insight of what it could mean or is just an irreverent anomaly that the 3.5 Om603 has? This car supposedly had an engine replaced 2 owners ago.