[ossiblue]

Here's what I can figure from the sources I looked at, regarding the engine:

64DXXX is correct for 83 model year US spec SC Targa, Coupe, and Cabio.

930/16 is correct for US spec SC's from 81 through 83.

The engine number on the COA has too many digits, unless there is a typo.

If there is a typo, it's likely the "0" after the "3" but there still is a problem with 6531810.

6=six cylinders
5=California specific engine
3=supposed to be the year of the engine but 83 use the letter D.

We've all seen the exceptions to every rule in the factory production of these cars, so nearly every source can be in error. However, given the reputation of COA accuracies as well as the incorrect number of digits in the engine number they sent you, I'd suspect a mistake.