[MrFixr55]

6ft sounds standard, and may be a NEC (National Electrical Code) specification. For a long time (at least back in 1952) the NEC had a spec where outlets had to be no more than 6 ft from any obstruction or break in a wall such as a corner, door, closet, stair, etc. and no more than 12 ft. from the nearest outlet. This is one reason that most outlets are not in a corner but in the middle of the wall.

The standard length of a lamp cord from the base of the lamp is 6 ft.

The reason why the cords that you are purchasing are 7 or 8 ft.. or longer is because there has to be consideration for the length of cable inside the chassis which can be as long as 1ft or longer in some radios. I am not sure if their is a spec as to from where to where the length is measured. For example for a console radio, the length may have to be measured from the plug to the base of the radio.

I tend to make some of my radios as having shorter cords as they sit on a shelf, connected to an outlet strip. However, for those radios that I used period correct cable (IE Cloth covered or twisted pair, I usually leave it the standard length.

Hope this helps.