Only one timebase option can be installed the 53131A (Option 001, Option 010, or Option 012).
The Option 012 timebase for the 53131A counter is customer installable. In order for the instrument to be retrofitted with Option 012 the instrument must have firmware revision 3402 or greater. To determine the firmware revision turn the instrument off, press and hold the RECALL key, and press the POWER button to turn the instrument on. The revision number will appear in the display.