Thetford O ring replacment

[CamperTom]

The pump for the Thetford cassette toilet in my QuickSilver 14XLP seized on me recently.  I was able to get the pump out and get the propeller free so the pump now works.  However when taking the pump out, when I pulled out the cover that seals the fresh water tank for the Thetford toilet, the O ring that was around it cracked and fell off.  I cannot seem to find information on the part number nor what size O ring it is so I can purchase a replacement.  Does anyone know the part number or size or a place to get a replacment O ring?  Since its broken into pieces its hard to get the size of the O ring.

In the attached picture, its part number 44.  However on the web site where I found it, they had no information on that part number.  Also Thetford has no information on the O ring on their site as well.

Thanks!