Springfield Marine's service part.
Spring-Lock Post End Cap
To install: Remove screw that is in the latch (do not throw away), remove old end cap. Press new end cap into bottom of post, install the latch back and insert the screw. If your post does not have the screw you will need to drill through the post and the latch and insert a #6 x3/4" Phillips Self Tapping screw. SKU #8570290
You may also need the post latch SKU #2100011