8 3/4 ear end ?

Getting ready to install a 8 3/4 rear end in my dart and I noticed the hole on the shaft housing didnt have a cap or bolt it's just a 1/4-3/8 hole no threads this is a vent ? where would locate a new one ? the disc that I have just said it's a "T" & a Screw please help Thanks !!!
Author: admin