Before January of 1958 SONY was the brand name used on electronic products produced by Tokyo Tsushin Kogyo.
After the success of their TR-63 pocket radio in the U.S. the company decided that changing the corporate
name to SONY would help in gaining acceptance world wide....it seemed to work
I have several TR-72 radios and all of them exhibit the same lettering which makes it appear like TR-7Z
but the radio you have is a TR-72.
The TR-72 was produced between 1956 - 1960, You have an early Tokyo Tsushin Kogyo version and judging
from the serial number yours may be from around late 1956. The early versions have the word TRANSISTORIZED
on the metal band that runs across the front/bottom and later models say SEVEN TRANSISTORS.
Here is a link to all the Sony radios in my collection including a few of the TR-72s. Lots more info therehttp://www.flickr.com/photos/transistor ... 964396197/