Hi Ian,
Under racing conditions it seems the vibration causes the metal part of the dip stick to break up where it joins the threaded top.I noticed mine was breaking up before it actually fell away!Don't use any more made up a alloy top to seal hole,and just use dip stick to check oil level.
If you run with out the balance shaft its more likely to happen!
I spoke to Richard Slater at the time who confirmed it happens.
If I was you I'd make up or get one made,just a skrew in top say out of alloy.
I lock wire mine,drilled a hole in the alloy ledge of the starter motor housing to wire it too.
Andy