I got to thinking, looking at my '76 project. with the old style loop Glow Plugs. They were pretty reliable weren't they? I know - if one opens up in the series connection they all go cold, but I never had one fail!
And regarding the pencil-style GP's there are two versions, one is 2 Volt the other 2.2 Volts. Does anyone know why Bosch made different voltages? For less cold climates perhaps? Or the 2.2 V plugs last longer? Just curious.

Dieseldiehard
1971 220 4-spd manual 104646
1979 300TD w/ ’85 turbo engine 290820
1983 300D 238890
1985 300TD 207980
1976 300D looking for an engine