According to the books Armalite manufacture that rifle from 2009 thru 2013. With the letter you have that puts it's manufacture in 2007.
I am not an expert on Armalite but I have always wanted a .338 Federal, so that is the reason I know a little bit about them.
I have seen a few gun magazine articles of hunters using them to good success in killing Big game animals. What is not to like a .308 family cartridge
(accurate and low recoil); big bullets travelling fast enough to expand, but not too fast to breakup on impact.
Plus it runs in a large frame AR, Soft shooting, accurate, parts are available, ammo is a little harder to buy off the shelf, but what ain't in these crazy times.
I hope you didn't pay to much for it, and I hope you enjoy it. Fjestad Blue Book has AR-10T listed so you can look the prices up there if you want.
I don't know how you could price a prototype rifle, but the letter you got cant hurt.
Enjoy and use it Well