When socialists went to prison they liked to pose later for 'prison' postcards.
In this postcard, one of The Melbourne Gaol Series, Tom Mann is picking oakum, a common prison activity of teasing apart old rope for ship caulking.
JPCML. Records of Robin Glenie . "The Melbourne Gaol Series" postcard - Tom Mann picking oakum, c 1906. JCPML00510/8