i am replacing the output shaft seal on my 2001 4r100 tarnsmission. i pulled tailshaft housing off to replace seal and bushing. when i got housing off i see a rubber doughnut on output shaft. this is on a 2001 ford lightning. i was wondering if anyone knows what this rubber doughnut ( for a better word) is for?... i do not see it on any parts lists

 

Keeps trans fluid in.