Yes, the RAG transposon inserted itself into a variable domain exon about 500 million years ago. Here’s a more recent paper on this:

Origin of immunoglobulins and T cell receptors: A candidate gene for invasion by the RAG transposon

https://pubmed.ncbi.nlm.nih.gov/40614193/

> It's the first time I hear this. Can't yet say much about it until I can verify that.

How are you going to verify it?