Not rhyming slang but interesting non the less:

"Piece" does not refer to a piece of bread (as stated above and often thought). Scot.s tend to refer to thier sandwich as a "Piece". This however derives from the practice of making a batch load of porridge, letting it solidify into a block and cutting a "piece" off to take out to work for the day. Although you may hear reference to "ma piece 'n' cheese" or "ma jeely (jam/jelly) piece" in place of a cheese or jam sandwich it should be taken as a reference to any portable meal.

Anyone know the origin of "messages" used instead of shopping "Ah wiz oot tae get ma messages"

PS There's a fantastic Glaswegian poem call "My Jeely Piece" about the arrival of the tower blocks in the Gorbals - a girl describes how her "Maw" couldn't throw her lunch out to her from the 15th floor of a tower block if she forgot to take it with her!