If you are talking about the build number, current build number is 6.5.1-167_DHD-14_m2-5. I don't know what the earlier build numbers are, but if you are looking to roll back to an earlier ROM, you may want either .173, .748, or .744 Each can be obtained by flashing the various roll-back ROMs found on DroidRzr (see link below), or you can flash Fastboot to .744 with the information found on XDA (see link below), and start the OTA update process from there.
This post on DroidRzr tells you the steps you need to take to either upgrade from 6.11.748 to 6.12.173 (earlier leaked version - now available as an OTA for most), or to downgrade from 6.12.173 to 6.11.748, or to downgrade from 6.11.784 to 6.11.744.
Links to the files (since it's a bit difficult to understand)
This thread at XDA will walk you through flashing the 6.11.744 Fastboot to take you back to stock. Once flashed, you will also need to do a Factory Reset to be completely stock and remove all user-installed apps and data. If you want to only flash back so you can get on the upgrade path again, don't do the Factory Reset afterwards, and also you will preserve all your apps and data.