Tissot balance wheel?

[Tricati15]

Hello. The pic is of a recent balance wheel with combined spring I purchased for my Tissot Seastar 781-1 movement. It differs from the one that was fitted in that, the stud at the end of the spring on the original assembly was smaller. The latest one  appears to have a screwed cap which, if I could remove, I`m sure would fit the where intended.  My problem is, I can`t shift what looks like the slotted screwed cap. Does anyone have any idea if indeed the cap is meant to unscrew?....and, the best method of holding it.  I`ve tried with an appropriate pair of tweezers without success.   

[rogart63]

Think there is 2 different versions of that balance and hairspring stud. There is a version that has an excenter regulator. That the model you got now. That balance won't fit the other version. 

Check http://www.ranfft.de/cgi-bin/bidfun-db.cgi?10&ranfft&&2uswk&Tissot_781_1