Mask was added with the SD, while the shield and Chobham armor opening functions for the Gatling guns was added with the MG 2.0. None of those functions existed on the Alex originally.

The SD also had this weird function where you could slap the leftover pieces of Chobham armor together if you weren't using it on the Alex to make a makeshift tank.

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Also I have to admit, that gatling gun popping out from the arm is one of the coolest designs out there.

What a lewd Gundam. /s