Wacom STU signature pads (such as the STU-430 and STU-540) can be used on Linux via either of the following SDKs:
1. The STU SDK for Linux
The installer for this is available under "Device kit" at https://developer.wacom.com/en-us/developer-dashboard.
2. The Ink SDK for signatures for JavaScript
This can be downloaded under "For signature" at https://developer.wacom.com/en-us/developer-dashboard