I too thought that the "50's" rounded corner look stayed around till around 1961. There is a transition period in production that allows the company to use up as much of the parts and warehoused/unsold stock. I am glad this came up because I have always been puzzled by the Harry Poster website.
For those who dont know Harry Poster does prop work for the movie industry and sells vintage sets that have been converted to color, or you can buy "as found". They list them in decade catagories, 40's through 80's. For a long time they have had a set in their 1970's listing (item#SC-SC01). It is a Setchell-Carlson that looks every bit a 1950's set with very rounded corners od the CRT.
Nefore I posted I went back to
http://www.harryposter.com/vintagetelevision.htm to make sure the ad was still there. It is. Check it out. Also now they have a rectangular set in their 1950's block (item#MAG-7Co1)
Everyone look and tell me what you think. The guy should know his stuff. Maybe he's letting the wife do the descriptions.