I'm upgrading my cBR8 from 17.9.1w to 17.12.1x.  My SUP ROMMON is 16.7(8r)S.  In Table 1 (or 3 depending on doc version) Firmware Versions it says the minimum required version is 16.7(9r)S so mine is a version older (16.7(8r)S).  But in the section "...