My pool was built in 1984. All our equipment is original. Our large filter tank has a drain plug at the bottom. It is leaking and I need a replacement but am having trouble finding this type and size. I have attached photos (as best as I could get them) of the white drain plug that exists now. 
I’d like to know the size and type so I can clean my tank easier.
Could you post a picture of the complete tank? Is there a multiport valve? If so, is there a model name or part number?