Watching Columbo and a headstone had the name "Slaughter" :)

Cool! I have the entire series. I'll tell you one I found... you remember the episode where the art critic kills his uncle? The studio he used as his alibi, the guy from Mel's Diner, artist Sam Franklin. Well, in another episode where Columbo is in a hospital waiting area, arguing with Robert Conrad ( the fitness guy ), well, on the wall in that scene is one of Sam Franklin's paintings. You can see the signature very clearly :-)

It was "Rest in peace Mrs Columbo". Just after the "funeral" for his wife he stops to talk to the murderer before getting into his car and that's where the headstone is. I can't find a picture of it, sorry. In "Murder by the book" (with Jack Cassidy) the murderer (Cassidy) gives the woman at the store a copy of "Prescription Murder" which was the name of the pilot episode :)

I do remember the book, that's a good Easter egg!
I think I remember an episode where there's an ad on the radio for Milo Jannus gym ... or something like that. Yes, I think you're right that these were placed on purpose :-)