This is a seal PSTN I/D for Velvet Drive 72 series transmissions, part number 1000-016-020. See the 72C series for more.
Item 721 on the Velvet Drive 5000V V-drive parts diagram (P-3 Input shaft). 2 per transmission.
Also comes in: 2002-410-001 gasket and seal kit.
See it on the parts diagram