Here is a plot of the five resulting fitted polynomials, superimposed on the data and extended to 1983. I have added a vertical line at 1982.25 so that you can see that the different fits give wildly different extrapolated values. You will not be able to see a difference between the degree 3 and degree 4 fits. Overall the degree 3 fit is probably best but does have a lot of parameters for the number of data points. The degree 5 fit is a statistically significant improvement over the degree 3 and 4 fits.
Extrapolation is very dangerous and unreliable.