You probably figured out what's up by now but thought to put my 2 cents out there:
When running "ver" on such gateways, before they are licensed, will indeed show 1550 being the appliance platform.
Once you activate your firewall, it "magically" turns into a 1535.
Here is another cent:
When running as cluster, make sure you have both firewalls activated and restarted before putting them in production.
While doing so later is definitely not a big deal, since they're running HA, it is less exciting doing so prior.
Reason is, once you have one of two activated and licensed, the cluster status will not be happy and complain about a core mismatch.
That can throw people off, when not aware.
Once you have the second member activated and rebooted, the the cluster is happy again.