Tried finding some information on the difference and why that is. Couldn't find a bead on it though. So um... why?
To clarify, Act labs made a dual gun system years ago called "GS". They are - IMHO - utter crap. Here is why:
The "GS" guns are much older. They come in a dual gun holster.
They also:
Do not work with Windows 2000
Do not work with Windows XP
Connect via the old Gameport connector, so you need a Motherboard with a 'gameport' connector.
*The do NOT work with Mame* (They use a proprietary protocol), no mouse simulation.
The USB version I am selling is plug and play in 2k/xp, and work perfectly with mame.