Firmware update Fortigate-600D

I inherited cluster Fortigate-600D from previous admins, which says that the Hardware version is FortiGate-600D Low-Encryption (LENC), and Firmware version is v5. But, first of all, I don't understand why everywhere I look (web, console) shows exactly v5, and not according to the normal type v5.0.2. Secondly, when I click check for updates, it says "No new firmware versions are available" there.
I could do a sequential download of each next version to gradually upgrade to the highest possible version, but first I want to be sure.