M+F is the ONLY factory that makes the FXD models.
The standard movement for the FXD is the Miyota 8215. It was chosen, because it's thin and fits into the equally thin Tudor FXD.
Very few people are crazy enough to swap the 8215 for an 9105. The primary difference between the two Miyota movements is that the 8215 is a 3 Hz movement, whereas the 9105 is a 4Hz movement, like the original Tudor.
In plain English, let's say you had sex with a watchmaker, and while you light up your cigarette in bed, she is staring at your diving watch which you refuse to take off, ever. While she watches the second hand sweep, she might notice that it is at a 21,600 beats per hour frequence, instead of a 28,800 bph.
In short . . . total bullsh*t.
However, once the 8215 fails, which is usually after 20 or 30 years, it makes sense to spend $15 more for the more premium 9105 replacement.