Just throwing this out there to see your opinions.
Typically I trust that the mainspring that I find in a watch is the correct one. However, I recently opened a watch and found that it was definitely not the correct mainspring. Someone hacked a T-end mainspring into a normal manual bridle.
I could not find any information about the movement online, so I endeavored to find out what the proper mainspring might have been.
Assuming that mainsprings are designed to follow the 1/3 are