None of the appliances I administer show any new firmware versions since January - the two at the site I'm currently visiting, for instance, are running 5.1.3.007 (2014-01-15), and both show this as the "Latest General Release" and report "No Early Release Firmware Available". Both are covered by current Energize subscriptions. I don't have specific details available for ones at other sites, but they all also have current subscriptions and show no firmware newer than the same or similar. Some are model 100, some are model 400.
All have shown, and been able to install, firmware updates in the past. I'd like to upgrade them to current firmware, but they won't let me.
I will place a guess that your serial number on the units is less than 200000. I don't know what the cut off is, but I can tell you I've admin'd units with serial numbers in the 120000 range, and even with current EU, you can't upgrade past 5.1.3.007. If you reach out to Barracuda, they will tell you that even though you have current EU, it isn't supported by the hardware version you have. Their solution is for you to pay 75% of list to purchase a "hardware refresh" which will give you a brand new unit of the same model, and they will transfer your existing subscription to the new device.
The primary reason they want you to hardware refresh, is because new hardware versions come with faster CPU's, and more RAM.
Having said that, you can debate your feelings all day about the fact that you are paying for current EU, yet can't really take advantage of it.