I would like to show you an example of this underprint on a stamp but, as yet, I have been unable to locate one.
This underprint is probably from the firm Welch, Margetson & Co. Ltd.
1868. Unofficial underprint type 42 (over the gum) in black.
a) Vertical underprint, reading upwards
PP139 2d blue (SG 45 or G2)
b) Vertical underprint, reading downwards
PP140 1d lake-red (SG 43 or G1)
PP141 2d blue (SG 45 or G2)
c) Underprint horizontal
PP142 1d lake-red (SG 43 or G1)
A single perfin has been identified as belonging to this firm. I do not have an illustration of the perfin but it is referenced in Tomkins as:
See the Philatelic Links and References page for more information on Perfins.