For Velvet Drive 72 series transmissions, this is the o-ring suction tube collar, part number 1000-141-214. See the 72C series for more.
Item 145 on the Velvet Drive 5000V V-drive parts diagram (P-1 Transmission). One per transmission.
Also comes in: 2002-410-001 gasket and seal kit.
See it on the parts diagram