Aaaaarrrrrrrrrrrrrggggghhhhhh!!!!. 'Step away from the bolt/screw minefield', Bert. lol.
The more research you do , the more confused you will get.
I did find this, 'The markings such as 7T (other common ones are 4T and 9T) are Japanese Industrial Standard (JIS) grades and indicate 1/10 of the nominal tensile strength in kgf/mm2'.
I've seen references to the 'T' meaning max torque but it's more likely to refer to max Tensile strength as Sye states and I guess was used up to the adoption of ISO heads and thread forms around 1971. .
Brian.