This rubber is measured by it's OD (outer diameter).
All other ring measurements other than this mini post are ID (inner diameter) measurements.
This mini post is slightly larger than the 3/8" OD mini post.
Williams #23-6694-1.
Sega/Stern/Data East #545-5348-17.
American Pinball #RBR0003-01.
Jersey Jack Pinball #25-006005-07.