These are internally different (using XTL-based boards) and require Astro 25 Mobile CPS (RVN4185) instead. You can identify a "Plus" model by its boot screen ("SPECTRA+") or a specific label near the model number. Hardware Requirements for Reliable Programming
Newer versions either hide these tabs or require a separate "dealer key" dongle, which is nearly impossible to find today.
The consensus among experts on RadioReference and enthusiast wikis like W3AXL is that is the best—and final—software release for the standard Motorola Astro Spectra.
Released in 2008, it was the last version ever produced for this specific radio line.