Ok... here is a graphical history of AGD markers courtesy of the Airgun Designs Virtual Museum.
Original RT Circa 1996
THings you should note here is the gas through RAIL! Often times there was a duckbill asa under the trigger frame and a line going from that to the foregrip, on the other side, there was a hard steel line going to the valve (the RT valve.)
If you have one of these, then there are only a few upgrades available. It is a workhorse of a marker and they look pretty cool in my book.
The RT PRO circa 2000
Here the gas through rail has been done away with, and the marker accepts all of the AGD and aftermarket upgrades.
The Current (user upgraded) AGD RT Pro ULE 2004
Here is a sweet mag showing many of the aftermarket parts available. Verticle Chimera Frame and splinter rail, by Roguefactor, X-valve, and gas through grip. A sweeter mech marker you will be hard pressed to find.