This is a Kadee #233 HO Scale 30-Series Gearboxes & Lids

#233 HO Scale 30-Series Gearboxes & Lids

Plastic 30-Series Draft Gearboxes, Draft Gearbox Lids and Spring Lids
10 pair per package

Draft Gear Box, Draft Gear Box Lid and Spring Lid for 30-Series Couplers.

Note: Uses #621 Torsion Centering Spring

2-56 screws are the most commonly used for center hole.
Screw length will very depending on the mounting situation.