You can kinda deduce from the serial number. Look for a 95 S350 on eBay with VIN 27 or greater. Get the engine serial number for the VIN from EPC. If the serial number of the engine on the 91 is close to or greater than the eBay engine serial number, there's a good chance it has updated rods. Mind you it's a leap to say the serial number of a 603.970 follows the sequence of a 603.971, but it's a start. It's enough of a leap to believe 95 SDs with VIN 27 or greater have updated rods. I haven't seen credible documentation.

Sixto
87 300D