I think the "apprenticeship" largely involves Stanley Robertson passing on his repertoire, and as Sam's activities keep him very busy in England, I think that at the moment this largely happens through Stanley recordng his songs and sending them to Sam.

Sam wears Stanley Robertson's sovreign ring, hwich he gave to Sam as a kind of token of the apprenticeship.

I'm not really aware of this as a common practice, but it's an interesting idea with regard to keeping the songs alive and out there, rather than simply "collected" onto recordings.