What it is
The 6ES7791-0DN00-0XA0 is a SIMATIC PC memory expansion module for RI25/45 Pentium systems, providing 8 MB of memory arranged as two 4 MB high-density modules.
Sourcing reality
No official successor order code is listed for this memory expansion, so a direct drop-in replacement from Siemens is not confirmed. A parametric search for 8 MB SIMM modules compatible with the RI25/45 backplane would be the practical next step.