pump for fountain / waterfall missing part

Hi there! I am trying to get my little fountain running properly and I came across this curiosity. It appears that there is some sort of adjustment piece that is supposed to go in the empty hole. I'd like to get a new one but I have no clue what such a thing might be called. Does this look familiar to anyone? Thank you!Attachment 38971 (http://www.doityourself.com/forum/attachments/ponds-fountains-water-gardens/38971-pump-fountain-waterfall-missing-part-img_1541.jpg)