We do not have an owners manual for the Hamilton Beach D50065 Commercial Coffee Pot. You may want to try to get one through the manufacturer.
It looks as if Hamilton Beach does not supply just the lid. You need to order the complete 990016601 Filter Basket W/ Lid.
Hamilton Beach does not provide internal parts for their appliances. You will need to take your Hamilton Beach D50065 (A-B) Commercial Coffee Pot, to an authorized Hamilton Beach Service Center.
If you go to the parts diagram for the D50065 below. And then click on Download Diagram, then scroll down all the way there is one part not shown on our parts diagram. It is part number 990092500 water window gauge gasket (But it is not black). If that is not it, I am thinking it is part of the 990044300 water gauge. If I am wrong again. You may want to contact the manufacture @ 800-851-8900. I am sure they will be able to help you.
Hope this helps,