That four digit stamping in the headbadge is the assembly date. It's a bit fuzzy for my bifocals to make out. The first three digits are the ordinal day of the year, the last digit is the last digit of the year. For example '3128' would be the 312th day of 1978 or 1988. You would have to determine the year model from the frame serial number (month/year of frame build) and frame style, components (date codes on some of these), colors, decal styles, etc.

Catalogs here:

http://www.trfindley.com/pg_schwinn_cats.htm

Bob

Edit: Doh! Pastor Bob types faster ...