the vintage cannondale site has a page which explains how to decipher serial numbers:
https://vintagecannondale.com/info/serial_numbers/

yours falls under "version 2"
02105911740
0 = 130 rear spacing
21 = frame size
0591 = date of manufacture May 1991
1740 = sequence number

https://vintagecannondale.com/year/1991/1991.pdf