as inspired by timcc23 post below.
I posted this comparison and my take on the matter last year on another forum.
The main an visible difference is the balance wheel and hair spring attachment. The 'B' has not only a different and adjustable stud(where the end of the spring is attached) but a new and also adjustable regulator. The regulator can be set for precise alignment with the spring which passes between it's bars/rollers and the space between the bars can also be adjusted. Like the regulator, the stud can be adjusted to take into account any slight variations in the coil wind and geometry of the hair spring. It is also suspected that a new material may be in use for the hairspring; one that could be more thermally stable.
These changes seem to have occurred around the same time as the 6r15B appeared. Perhaps both movements now enjoy the same parts
Other than this, I don't think anyone has discovered other differences between the 'A' and 'B' versions of either caliber.
Picture courtesy of AndrewB
One of my own 7s26 movements.