The stranger turns out to be Del Boy and Rodney's father (and Grandad's son) Reg, who had abandoned them 18 years earlier after the death of their mother.
Rodney reassures Del that children with the same parents can have different blood groups and recommends that he visit Dr Becker, the Trotter family's doctor, again to clarify the results.
It is when Reg taunts Del yet again by calling him "Ke-mo sah-bee", (another Lone Ranger reference), when dealing him his cards for a game of Pontoon, he changes his mind.
Furthermore, Dr Becker made some background checks, which found that Reg was actually a porter at the Newcastle infirmary, not a patient, and is on the run after stealing numerous items from the hospital, including the chief gynaecologist's Lambretta scooter.
The manner of Reg's departure meant that John Sullivan thought it impossible for the character to return because "he'd blotted his copybook so badly that I couldn't see Del ever accepting him again".
Following Lennard Pearce's death less than a year later this exchange was removed from the episode, with it not being restored for any subsequent airing or home media release.