“Brass band with a leader” was lunch counter slang for “pork and beans.” “Brass band without a leader” was slang for “beans” (without the pork).
 
“For instance, the succulent bean, the food of the demigods who people Boston, is known as ‘dynamite.’ When pickled pork is desired the combination is known as ‘brass band, with a leader’” was printed in The Daily American (Nashville, TN) on April 13, 1887. “‘Brass band, without a leader’ is a plate of beans without pork” was printed in the Brooklyn (NY) Daily Eagle on July 3, 1887.
   
The “brass band with/without a leader” restaurant slang terminology was rarely used after 1910.
   
   
13 April 1887, The Daily American (Nashville, TN), pg. 6, col. 6:
A BRAKEMAN’S DINNER.
Edibles by Names That Floored Even the Ubiquitous Reporter
Railway Reporter: ...
(...)
For instance, the succulent bean, the food of the demigods who people Boston, is known as “dynamite.” When pickled pork is desired the combination is known as “brass band, with a leader.”
   
Brooklyn Newsstand
3 July 1887, Brooklyn (NY) Daily Eagle, “Restaurant Calls,” pg. 13, col. 1:
“Brass band, without a leader” is a plate of beans without pork.
   
22 January 1888, Omaha (NE) Daily Bee, pg. 3, col. 5:
IN AN UNKNOWN TONGUE.
Waiters Who Serve Customers in a Mysterious Manner.
(...)
Pork and Beans—“Brass band with leader.”
   
4 April 1897, The Morning Sun (Chanute, KS), “Slang of the Restaurants,” pg. 3, col. 2:
“Brass band with leader”—pork and beans.
 
2 December 1902, Plainfield (NJ) Courier-News, “Short Town Sketches,” pg. 2, col. 1:
It was a cold, undiscriminating waiter in a New York restaurant the other day who took the order for a meal from a couple of Dunellen gentlemen.
(...)
They were considerably chagrined when they saw the waiter size up their seedy clothes, and were rather surprised when he called out “brass band with a leader,” and returned to them with a dish of pork and beans.
 
Chronicling America
24 July 1909, Norwich (CT) Bulletin, pg. 4, col. 3:
Every new calling seems to bring out new phrases. It is not so strange that we should have to go to Massachusetts to learn how to order lunch cart beans. As an order of plain beans do not make an attractive sound; but when a hungry soul enters a night lunch cart and says “1,000 on a plate,” everybody takes notice. There is something broad and beaming about that order, and it only means a plate of beans. But pork and beans is too popular and cheap a refreshment to let escape with just one style of verbal embroidery, for some restaurateurs recognize it as “a sheet or music and the instrument” or as “a brass band with the leader.”
 
Google Books
Cassell’s Dictionary of Slang (Second Edition)
By Jonathon Green
Londeon, UK: Weidenfeld & Nicolson
2005
Pg. 175:
brass band with a leader n. [late 19C] (US) pork and beans; thus brass band without a leader, beans without the pork.