The O.S Type F Glow Plug - for Four Stroke use. Not for just in O.S four stroke engines, but many makes of Four Stroke engines can use a type F.
A top quality plug. It does exactly what it says on the packet !
Glow plugs are considered to be consumables - But by using a low voltage to start ( under 1.5 volts) and not burning the plug dry, you prolong the life of your glow plug. It is also worth washing out a new engine after running in to prevent small particles shorting the plug and destroying it. Blowing plugs on new engines is less common now a days, but is still something that can happen.