My '50 has the same hole. Please see the below link from Jim Carter's for information about the fifth bolt hole. And the rare filler bolt that can be installed in the hole.

http://www.oldchevytrucks.com/cgi/htmlos.exe/002310.2.1917951597600010861

I have been trying to locate
one of those filler bolts for thirteen years. I guess I will have to get used to the hole.