“Knock, knock.” / “Who’s there?” / “Diploma.”

“Diploma” can sound like “da plumba” or “the plumber.” A knock-knock joke is:
 
“Knock, knock.”
“Who’s there?
“Diploma.”
‘Diploma who?”
“Diploma is here to fix the sink.”
 
The joke has been cited in print since at least 1984.
     
   
Google Books
Laughs, Hoots & Giggles
By Joseph Rosenbloom
New York, NY: Sterling Publishing Company, Inc.
1984
Pg. 118:
Knock-Knock.
Who’s there?
Diploma.
Diploma who?
Diploma to fix the leak.
 
28 January 1996, Newsday (Long Island, NY), Kisday sec., pg. 1:
Knock, Knock: Who is there?
Diploma.
Diploma Who?
The diploma is here to fix the boiler.
Jim McNey, Bayside
 
Google Groups: comp.sys.hp48
Question About Fractions…
Chrstpher
5/19/98
(...)
Knock, knock.
Who’s there?
Diploma.
Diploma who?
Diploma is here to fix the sink.
 
Twitter
LITTLE MISS PATATAS
@iamjanemariano
KNOCK KNOCK! ~ WHO’S THERE? ~ DIPLOMA. ~ DIPLOMA WHO? ~ DIPLOMA’S COMING TO FIX DE SINK.
6:18 AM · Jun 19, 2010·Twitter Web Client
 
Twitter
Roger von Oech
@RogervonOech
Knock, knock.
Who’s there?
Diploma.
Diploma who?
Diploma is here to fix the sink.
6:42 PM · Dec 3, 2010·Twitter Web Client
 
Twitter
daisy
@aidazmn
‘knock knock’ ‘who’s there?’ ‘Diploma’ ‘Diploma who?’ ‘Diploma to fix the leak.
2:34 AM · Jan 7, 2012·Maxis tweet2me
   
Google Books
The Jokiest Joking Knock-Knock Joke Book Ever Written…No Joke!:
1,001 Brand-New Knee-Slappers That Will Keep You Laughing Out Loud
By Brian Boone
New York, NY: St. Martin’s Publishing Group,
2018
Pg. ?:
Knock knock! Who’s there?
Diploma. Diploma who?
Diploma is here to fix de sink!