I think you may be mixing two companies together here. Sheffield Steel Products Limited existed before hand and had a trade mark SSP. It was not unusual for them to produce tools, or blades of forged steel, for use in other products, and included the makers name in their die stamp on the metal component. It would probably have been sensible of Thomas Firth to have outsourced the early production of his blades to them. I can remember these knives in particular from my childhood. They were hard to keep sharp and the handles chipped easily. I liked them though!