I believe that the change from the instrument panel mounted light to the dash rail mounted light was made in mid-1930. 1931 should have the dash rail light and should not have a hole in the panel. My early 30 sedan has the panel light, and my mid 30 pickup and early 31 sedan have the dash rail light.
You should have a round speedometer for a mid 1930 and 31 vehicle. If you have an oval speedometer in your 1931 Model A, them someone put an early 1930 gas tank for an oval speedometer in your coupe. Here is a picture of a mid 1930 - 31 dash and round speedometer.
A picture would help a lot.