"Smithfield pens" likely refers to working at the Smithfield livestock market in London. The Smithfield Market is one of the oldest markets in London, founded in the 10th century for the selling of livestock and later meat and poultry. It is located in the Smithfield district of the City of London. The present covered market on the site was designed by Sir Horace Hones and completed in 1868.

In London Labour and the London Poor:

Meeting of Thieves. (Volume 1)