I know in the early 90's at least, the 4-digit designations seemed to indicate non-traditional tube materials; i.e,. aluminum, carbon fiber composite.
1400 Aluminum (Road)
1420 Aluminum (Road)
1000 Aluminum (Road)
1100 Aluminum (Road)
1200 Aluminum (Road)
2100 Composite (Road)
2300 Composite (Road)
5200 Composite Competition
5500 Composite Competition
And of course the T-prefix was Tandem.
Not sure the span of years for which this is valid.
http://www.vintage-trek.com/images/t...ikecatalog.pdf