“A fire hydrant has H2O on the inside and K9P on the outside”

“A fire hydrant has H2O (water) on the inside and K9P (canine pee) on the outside” is a joke that has been printed in several joke books. “If you find H20 inside a fire hydrant, what do you find outside? K9P” was cited in print in 1989.
 
   
Google Books
Painfully Gross Jokes (Volume 8)
By Julius Alvin
New York, NY: Kensington Publishing Company
1989
Pg. 70:
If you find H20 inside a fire hydrant, what do you find outside?
K9P.
 
Google Groups: alt.tasteless.jokes
h2o
Shiloh Shorn
4/24/98
If you find h2o on the inside of a fire hydrant whadya find on the inside
k9p
 
January 2001, VFW (Veterans of Foreign Wars Magazine), “Jest a Minute,” pg. 50, col. 1:
I was asked what is the chemical symbol for water inside a fire hydrant. I said, “H20.” Then I was asked what is the chemical symbol for water outside a fire hydrant. I said, “K9P.”
Jim Tolnay, Corvallis, Ore.
 
Google Groups: alt.tasteless.jokes
If there is H2O on the inside of a fire hydrant, what is on the outside?
G - P
4/8/01
K9P
 
Google Books
A Prairie Home Companion
Pretty Good Joke Book
Introduced by Garrison Keillor
Minneapolis, MN: HighBridge
2003
Pg. 20:
If H20 is on the inside of a fire hydrant, what is on the outside?
K9P.
 
Twitter
Sam C
‏@Samography
ori asked for a tweet: If H2O is on the inside of a fire hydrant, what’s on the outside? K9P, of course!
5:04 PM - 31 Jul 2007
 
Google Books
Jokes for Blokes:
The Ultimate Book of Jokes Not Suitable for Mixed Company
By Llewellyn Dowd and Phil McCracken
New York, NY: Ebury Press
2011
Pg. 161:
If there is H20 on the inside of a fire hydrant, what’s on the outside?
K9P.
 
Google Books
Extremely Embarrassing Dad Jokes:
Because Dads Don’t Know When to Stop
By Ian Allen
New York, NY: Pavilion Books
2015
Pg. ?:
If a fire hydrant has H20 on the inside, what does it have on the outside?
K9P.