preglow relay replacement

I am looking for a replacement for the preglow relay on my 1979 240d.
The catalog from Professional auto parts, listed the preglow relay, silver colored, just as mine is, for the 616.912 engine at $114.
above it, was another one, which stated that it superceded the previous one. This one was black and included 4 glow plugs, pencil type (looked like pencil). For $91.
Obviously, I would like to replace the relay and get 4 glow plugs for a cheaper amount.
Question:
    Does my 1979 ( made in 10/78) have the loop type or the pencil type glow plugs?
    How can I determine which one I need?
    Are they interchangeable?
    Can I use this newer preglow relay?
    Any advice?
    Jerry
    
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You may be able to retrofit to a Beru or Bosch extended glow relay that continues the glow after the engine starts to improve combustion and so reduce smoke. Check their websites.
http://www.beru.com/english/produkte/gluehkerzen/nachruest.php
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