Posted by Eric B. [99.100.188.129] on Saturday, July 12, 2014 at 16:46:37 :
Here is the white faced 2230193 version that someone has listed on eBay. Link to the listing below.
You most likely need the black face with the white lettering which is part number 2507112.
I found the following NOS suppliers listing the later black faced gauge:
http://www.arizonaparts.com/
http://collectorsautosupply.com/
http://www.dpmotorparts.com/
I'd invest in a correct parts book for your 1966 W100. They are invaluable when trying to find a part like this as there are a lot of gauges in that era and only one is correct for your truck.
Good luck,
Eric