Both your Montana 680 and GPSMAP 276cx have protected firmware but it's of the 'weak' type so you may use unlocked CN IMGs but not unlocked CNU (Unicode) IMG files unmodified. However, Unicode maps with the code page changed to 1252 and unlocked with gimunlock V0.04 will work when MSV is 'weak' as in your devices. To use Unicode maps with original Code Page 65001 you'll need to patch the firmware using Big Bang's Universal Firmware Patcher, there's then no need to change CP to use unlocked NTUs.

A word of warning about patching Montana 680's firmware. You must reset the unit and disable Geocache Dashboard in map settings before using any unlocked map or the unit locks up when it is selected.