I had bought a real deal USGI tear drop forward assist from Numrich a while back and I ended up selling that upper. Big mistake, trying to build another one now.

Still Numrich #1226810 for stripped unless they have run out and started selling DPMS which they are known to do.   You had to swap in pawl, springs, pin and detent.  The last stripped USGI I bought probably 2 yrs now in caged bag.   Most all the same internal parts.  Not like Colt actually made them anyway.  Complete try GB seller MX-56 for USGI. Personally I'd get BM from Brownell's for $7 list is $8.99 less spring and pin.

Sarco Inc may sell USGI tear drop # AR049 but probably current production also.

Only reason I was looking for one of those USGI 1966 marked ones was the part as very good quality. The fit and finish was great. I had ordered a DPMS one once, it was crap compared to the USGI one I had gotten from Numrich a year or two ago.

Fulton Armory has them too!

Yep. I have a couple of them that I got from Fulton and even though they're probably of recent manufacture, they sure do look good.
They have a much smoother finish than the images indicate, and they're a very clean casting. I'm happy with them.