This accessory is required for the Quick-Seal Polypropylene Ultracentrifuge Tube Kits (part # 344622).
Spacers for Tubes in Beckman Coulter Rotors for use in Ultracentrifuge Rotors:
Type 90 Ti (part # 355530)
Type 70.1 Ti (part # 342184)
MLA-55 (part #s A31459, 393203)