Strange plugs which are interspersed with glow plugs.
 

Dear friends:

On my 1981 240D, I saw 2 strange plugs which are connected
to the same engine side (right facing the front of the engine) as
the 4 glow plugs. You can see them clearly as 1 and 2 on the attached image. What are these 2 strange plugs and what are their function?

BTW, the plug #1 has an attached connector while the plug #2
doesn't have anything. Am I missing some connector here?

Thanks a lot for your help.

Best regards,

Eric