This site
http://sixtiestv.com/color_tv.htm
says that in 1964 that only 137 out 565 commercial stations in the U.S. could originate color - often only film. So, the number of Tk-41s built may be much smaller than the 1000 maximum I estimated. It also said that NBC was scrounging color gear from such places as the New York World's Fair to supplement its equipment in the 1964-65 season.