While troubleshooting an issue this morning I went to upgrade the routerboard firmware on a CCR after bringing it up to 6.42. The upgrade-firmware now appears to match with the router-os version.
/system routerboard print
routerboard: yes
model: CCR1016-12S-1S+
firmware-type: tilegx
factory-firmware: 3.22
current-firmware: 3.41
upgrade-firmware: 6.42
Now, if Mikrotik would just provide release notes on the routerboard firmware in a handy place.