Saebo SaeboFlex Manual page 8

Step #8:
To apply the thumb digit cap, unhook the thumb spring
attached to the thumb mount on the forearm shell. Let the digit cap
hang down toward the thumb.
Step #9:
Slide the thumb into the digit cap. Make sure the tip of the
thumb is at the end (tip) of the inside portion of the digit cap and no
space can be seen.
Step #10:
Once the thumb digit cap is securely on the thumb, re-
attach the thumb spring onto the thumb mount. Be careful not to
overstretch the spring.