Anchor points in celing for carting surfboards inside Maxi

[jmd]

I plan to carry 6 - 9 foot surfboard (s) inside my 2010 Caddy Maxi. On both sides of the ceiling at the rear of the van, and behind the second row of seats, there are plastic caps (approx. 4 x 4cm) that I have removed to reveal metal with one threaded and one simple hole (about 3mm diameter).
Can anyone tell me that these are for?
And what size bolt would fit the thread?
My idea is to fix a loop here that I can anchor a strap that will act as a sling to support the surfboard.

[jmd]

Here are photos of what Im trying to describe in first post.

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[jmd]

photo of what im trying to describe

Sent from my Pixel 6a using Tapatalk

[jmd]

Turns out the threaded holes take an M8 bolt.

[jmd]

https://www.screw-express.com/lifting-e ... eQQAvD_BwE