They were most likely a dealer installed option. Diesels commonly come factory installed, but gas engines don't usually come with.
That NOS one is cool. A current manufacturer of essentially the same thing (but with grounded plugs) is Kats. [url=http://www.fivestarmanufacturing.com][/url]


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.