Using a psycopg2 converter to retrieve bytea data from PostgreSQL

I have used psycopg2's adapters and converters in the past with numeric[] arrays in PostgreSQL, and now I'm trying to use them with binary data types (bytea). I've outlined my problem in detail with some test code which illustrates what works (without adapters/converters), and how it fails when I build my working code into using an adapter and converter:

Would you mind taking a look and providing some advice?

Thanks,

Andy