I recommend using the pen that the manufacturer made for that tablet. The Wacom Pro Pen 1 works with the Xencelabs tablet, but you might sometimes have issues with tilt and/or pen pressure.
The Grip Pen (KP501E2) works, but pen pressure is not as fluid as with the Xencelabs 3-Button and Thin Pens.
On the Xencelabs website it mentions that the The KP503E Pro Pen 1 is compatible with the Xencelabs tablets. Is the tablet also compatible with the Wacom KP501E2 Grip Pen and what are the know issues with this pen?