Chrysler Marine called them “thrust collars”. They fit between the intake engine block and the distributor and raise the distributor up ¾”. They were necessary on the starboard engine of twin-screw boats and in any engine that had the rotation reversed from automotive. The reason is that when you reverse rotation, the distributor thrust is upward rather than downward and distributors aren’t designed to handle upward pressure, nor are oil pumps.

The port, or standard rotation engines like automotive, doesn't need this thrust collar and uses a distributor length of 4” on 440s. Distributor length is measured from the mounting flange on the distributor to the bottom tip driving the oil pump. The reason Chrysler Marine used it was to keep marine distributor lengths all the same at 4 ¾”. Necessary on reversed engines but not necessary on standard engines without thrust collars.

AAA MARINE PARTS WHOLESALE ( usually has both, and you can mix them or use the thrust collar on both engines and 4 ¾” distributors-it’s your choice.