I can't tell you the exact production date, but I have one with serial #G563xxx that I purchased in the mid 1970s, so I would think your gun was made in the early to mid '70s.
The "A" designates the gun is a 12 gauge. The "T" stands for trap. Any questions about Mossbergs can be directed towards Vic Havilin at www.mossbergcollectors.org or call him at 636-937-6401.
$145 nooo its worth alot more then 145....my dad has a 12 gauge mossbergg 500at...they dont even make an AT anymore my dad took his to a gun shop to see how much its worth and for the condtion his is in its about 600-700 and with the slug barrel and scope its worth about 800-900