This underprint is believed to be from the firm G. Hitchcock, Williams & Co. They were manufacturers of Mantles, Costumes and Millinery.
They operated from:
72 St. Paul's Churchyard,
A Post Office Commercial Directory of 1865 lists them as:
“Hitchcock (George), Williams & Co. warehousemen, wholesale & retail silk mercers, linendrapers, shawlmen, hosiers, haberdashers & lacemen, 72, 73 & 74 St. Paul’s churchyard EC; 45 & 46 Paternoster row EC, & 1 & 2 Londonhouse yd EC”
1866. Unofficial underprint type 65 (over the gum) in black.
a) Vertical underprint, reading upwards
PP218 1/2d rose-red (SG 48 or G4)
PP219 1d lake-red (SG 43 or G1)