“What’s the difference between roast beef and pea soup?” (joke)

An old burlesque gag is: “Q. What’s the difference between roast beef and pea soup? A. You can roast beef (but you can’t pee soup).” The joke is also given as: “Q. What’s the difference between mashed potatoes and pea soup? A. You can mash potatoes (but you can’t pee soup).”
 
The food joke has been told since at least 1954 and was used in the 1980s Broadway burlesque show Sugar Babies.
 
     
Google Books
The steam trawler Uganda: a novel
By Bevis James Taylor
London: Dent
1954
Pg. 30:
“Now then—now then—what’s the difference between roast beef and pea soup—now what’s the difference between roast beef and pea soup? Well you can roast beef.”
   
Google Books
Commercial Car Journal
v. 87 - 1954
Pg. 54:
First Diner Waitress: “What’s the difference between mashed potatoes and pea soup?”
Second Diner Waitress: “Anybody can mash potatoes.”
 
Google Books
The American burlesque show
By Irving Zeidman
New York, NY: Hawthorn Books
1967
Pg. 112:
“What’s the difference between mashed potatoes and pea soup?”
 
“I don’t know the difference between mashed potatoes and pea soup. What is the difference between mashed potatoes and pea soup?”
 
“The difference between mashed potatoes and pea soup is that you can mash potatoes but you can’t…”
   
Google Books
Sugar Babies: the burlesque musical
By Jimmy McHugh; Ralph G Allen; Harry Rigby; Dorothy Fields; Al Dubin; et al.
New York, NY: Samuel French
1983  
Pg. 83:
CHARACTER. (trying to get her attention) Madame. Madame. What’s the difference between mashed potatoes and pea soup?
GAZAZA. I don’t know.
CHARACTER. Anyone can mash potatoes.
 
30 October 1986, Los Angeles (CA) Times, “A top banana glories in classic burleque” by Herman WOng, Calendar section, pg. 1:
Second banana: What’s the difference between mash potatoes and pea soup?
Top banana: That’s easy. I can mash potatoes.
 
Google News Archive
31 October 1986, Sydney (Australia) Morning Herald, Metro section, pg. 5, col. 1 ad:
What’s (to difference between mashed potatoes and pea soup?
Anyone can mash potatoes.
 
BBC News
Thursday, July 8, 1999 Published at 12:38 GMT 13:38 UK
Entertainment
Your jokes of the century
(...)
What is the difference between roast beef and pea soup?
Most people can roast beef…
Keith C Franklin, UK
 
Google Books
Vaudeville humor:
The collected jokes, routines, and skits of Ed Lowry
Edited by Paul M Levitt
Carbondale, IL: Southern Illinois University Press
2002
Pg. 225:
What’s the difference between roast beef and pea soup? Anyone can roast beef.