For completeness, here is the default mappings when the trackball is seen with 11 buttons:
1 - Left Big (LB)
2 - Left Big + Right Big (M) at the same time
3 - Right Big (RB)
4 - Not used
5 - Not used
6 - Left Small (LS)
7 - Right Small (RS)
8 - Not used
9 - Not used
10 - Not used
11 - Not used

To get paste on LS here, the command will then be:
xmodmap -e "pointer = 1 6 3 4 5 2 7 8 9 10 11"