Richard,
Okay, so the sandbox doesn’t have SSO set up but production does. I checked my user record on production and yes, the external ID is set, and it is, in fact, a numeric ID. When I tested with the production credentials and URI and the real external ID, it returns the same 404 error.
Oh, and I’m not sure how to test SingleSignOnRecord.find_by(external_id: ‘mark89’). Where is this facility?
Mark