Haven't done one yet , I bought the kit from Cummins , from what I've read & done on other engs. , some others you can put on from the outside , after removing dampner , on the Cummins & some others you have to take the cover off [ kind of a pain ] , I do not think that mine came with a ring , but my guess is that it is used to keep the seals from getting damaged while your guiding the ft. cover back into position .
Many are able to save the cover gasget & reuse , but if your in a bind have one ready .