Cocker Brothers Limited were Spring Manufacturers in Sheffield.
1874. Unofficial underprint type 10 (over the gum) in black.
PP21a 2 1/2d blue (SG142 or J21)
There are a number of later perfins that have also been (provisionally) identified as belonging to this firm. I do not have illustrations of these perfins but they are referenced in Tomkins as:
See the Philatelic Links and References page for more information on Perfins.