Mounting artillery on a DropShip leads to the wrong date for Availability in Preview

[assaultdoor]

On MegaMekLab 0.47.6 on OS X with Java build 1.8.0_51-b16, the Summary tab in Preview shows the wrong dates for Era when the DropShip mounts artillery.

To recreate:

1. Create a new DropShip and set its year of introduction to some point prior to 3071. In this example, it's 3045.
2. Check the Summary tab of Preview. It says the vessel is Standard tech 3045+, which is correct.
3. Add a Long Tom to and assign it a firing arc.
4. Check the Summary tab again. Now it incorrectly says the vessel is Standard tech 3071+, and apparently does not exist 3045-3070.

The same results come from mounting a Sniper or Thumper. If you instead mount an Arrow IV, it is listed as Advanced 3045-3070 and Standard 3071+