According to the Ericson manual:
"The first three letters "ERY" designate the manufacturer, Ericson Yachts.
The next two numbers "28" designate the model and
the following three numbers are the hull number.
Following the hull number is a letter which corresponds to the month of manufacture where, A=January, B=February, C=March and so forth.
The first of the last three numbers is the last digit of the calender year that your boat was built and the last two numbers represent the model year".