In cases where generic non model specific plates are used the 4 digit number is just the consecutive number of that plate which could have made it on possibly 10 different models.- JEM plates are 6 digit and the first 2 digits is the production year while the last 4 is the consecutive number JEM built from inception. Except for the very first batch of 90' UV plates that began 00, UV plates are also dated by the first 2 numbers. A Universe plate 912787 would denote a production year of 91 and the 2787th Universe built.
The first digit will be the last digit of the year, ie. The last 5 numbers will be the consecutive number denoting production count for the year with the caveat that each month will start at predetermined intervals of 3000 or 5000, etc.
If you're trying to figure out what model something is from the serial number, figure out what year it is then go to the catalog library for the corresponding year [or year before or after] to find the model.
Some models will never be found in the catalogs, that's just the way it is.
In a case where a guitar has both the true serial number will always be the headstock number.
Plate numbers usually follow the pattern of the first 2 digits denoting year of production followed by a 4 digit number that denotes consecutive production for that model for that year, or consecutive production number for that particular model from the models inception.