[69L72RS]

There are seven different fill plugs if I remember correcly.
The open rear plugs did not have a flange between the head and threads.
The posi plugs had a flange between the head and the threads to hold the posi tag.
Here goes,
Early from 65 to 68 These had large plug threads and a square boss drive (I think the same size as the Corvettes).
Midddle from 68 to somewhere in 70. These had smaller threads and a square boss head.
Later 70 to ?. These had small threads and a hex socket drive.
Still later ?. These had a square socket drive and no flange. The posi tags should be metal and attached to a cover bolt.

Please, if somebody sees and error, post a correction. Going from memory and trying to stay awake. Not a cooperative combination.

Also, I am fairly sure that the 10-bolt and 12-bolt are interchangeable for the same year.