I was always thinking it was 76 but I may be putting too much stock in the number on the car?
I will concur with 1976 for the time frame. Here is my reasoning:
First photo (body)
From a 1975 Buzz A Rama entry form, I was using a Shadow body like this one
The side dams are stapled full length which is mid 1970s style.
Second photo (frame)
Faas gears, Bullit Heat sinks, L front axle, Orange tires all period correct.
The holes in the can have been enlarged which was occurring at that time.