The Pro Shields seal the Glowforge Pro so that neither light nor air can pass in or out of the passthrough slots. Professional laser safety officers advise that, with both Pro Shields in place, you may be permitted to operation your Glowforge Pro without further safety precautions. Both front and rear Pro Shields must be used; this is the rear part only.
Installation instructions: https://support.glowforge.com/hc/en-us/articles/360033633594-Remove-and-Install-Pro-Shields
This is an OEM part directly from Glowforge.