Jonathan McDowell's tweet
Six more debris objects from Transtage 17 cataloged. Here is the Gabbard diagram showing apogee and perigee for each piece - green line is the GEO altitude
led me to various pages about what a Transtage is:
Question: What is the large, conical, separatory funnel-shaped object reaching an apex at the aft of this upper stage? Do more recent upper stages have something similar?
below: rotated, from here.