What you're specifying
The Mitsubishi RD78G16 sits in the MELSEC iQ-R family as a PLC expansion module, motion-class, used to extend an iQ-R base rack with high-end motion control capability that the standard CPU cannot carry on its own. Catalog title and part number are both RD78G16; the listing carries it under PLCs and Accessories, sub-class PLC Expansion Modules.
Lifecycle stage is recorded as current, so the module is ordered against a normal factory quote rather than chased through surplus or broker channels. No successor order code is on record for this MPN; for a design refresh, evaluate current iQ-R motion module variants listed in the catalog. Specify the RD78G16 into the BOM and quote against an RFQ — that is the cleanest path while the part remains current production.