All Ruger firearms made in 1976 (I was in the 5th gread) were marked "Made in the 200th year of American Liberty".
They are really not worth more then other Rugers, but when you have an unfired one, in the box, with papers and original receipt, You have one that some people will be willing to pay more money for.
About 16 years ago I was given an unfired Mini 14 that was made in 1976. It was in the original box but, the box was in poor shape. There wer no papers in the box and the rifle had been drilled and tapped for the B-Square scope ring rail that mounted on the top of the resiever. The rifle looked brand new but had no collector's value. It has been a fun rifle to shoot. It does have a spl. place now. I gave it to my daughter on her 14 birthday. You should have seen the look on her face.
If I were you I would not fire the gun and take good care of it. With every year that goes by, it will be worth even more.