I have 4 old-style green porcelain lamp shades, 18" diameter. They are missing the screw-on piece at the top to which the bulb is installed and from which a conduit hanger is attached. The diameter of these threads is approximately 2 3/4". I'm looking for a source for these parts. Thanks for any help!!

I've seen some that are similat to what you're describing and I'm not sure if I remember right but it seems like they were made by crouse hinds. Hope this helps and that I'm not steering you in the wrong direction.

CF, are these what you're talking about? http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140064505307&ssPageName=ADME:B:EF:US:2#ebayphotohosting
IF so do some more searching on e-bay. I saw some others there for a little more money. After some reflection I now think that they were made by GE or Westinghouse, not Crouse Hinds.

I have four of those myself, exactly like the ones on ebay. They screw perfectly into a vapor proof fixture base, but I too would like to find the original type parts.

FWIW the Applebees resteraunt here uses a similar fixture so someone is still making them so if you're interested in new they're still available.