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