Those are the new updated plugs, not the series loop-type.
The good thing about your plugs is that if one goes out, you still have others that can warm up. With the series type, if one goes out, it breaks the circuit and none of them will glow. The series type do not ground individually on the engine, that's why they have ceramic insulators (which you do not have, sorry for the extra info.)

Your plugs each operate independently so just pull them, check the wires to each, measure their resistance and possibly ream out the holes. While the updated plugs are good for operation they can be bad in a way because they never tell you when you have a problem. If 3 out of your 4 plugs are bad, the indicator light will still come on and you may or may not get your motor started, so it kind of gives a false positive to the driver, saying that the glow circuit is all working.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson