Well look what I found... more typos.
[AGGattlingx] and [ElectricFragment] all have
Projectile=Invisiblelow
By all means, that should be InvisibleLow. That's Westwood's mistake, which even Cannis accidentally copied into his Gattling Boat([GattBoatAGx])
From what I've seen in PaD's decompiled code, only the generic prerequisites, such as POWER, can be miscapitalised, everything else is case-sensitive.