SPG? Not certain on this but maybe Tony Edwards provides a clue.
From Tony Edwards’ British Military Small Arms Ammo sites.google.com/site/britmilammo/-303-inch :
“Although the RL tracer had fulfilled a need, there was soon a demand for a more effective tracer. Aerators, a company best known for their “Sparklet” soda siphons were making 8mm solid bronze bullets for the French and from this developed a .303 inch tracer round which in trials at Hythe traced to 1,000 yards. “Cartridge S.A.Tracer SPK .303 inch Mark VII.T” was approved on 23rd June 1916 bit was not shown in Lists of Changes. A nitro-cellulose version “Cartridge S.A. Tracer .303 inch SPK (VIIT) NC Mark Iz” was approved in February 1917. The round was generally referred to as the “Sparklet” tracer”
“The Mark VIIT proved difficult to manufacture, particularly with respect of ensuring the tracer cavity was bored centrally so Royal Laboratory Woolwich developed a new bullet that used the same tracer compound but in a copper canister. “Cartridge S.A. Tracer SPG .303 inch (VIIG) Mark Iz” was approved on 18th October 1916…”