One interpretation is 6th month employment anniversary; Another interpretation is 180 days, counting 30 days for each month. But which one is correct?
Extension of probationary period is possible but not the general rule, lest it becomes a ploy to take advantage of the employee.
Ending endo is akin to relationship status- it’s complicated.
In your case regularization happened regardless of the lack of regularization contract.
One mode to non regularize a proby employee is to let the probationary contract expire, according to this SC decision