bljnv,
No, we are using the word "mounting" and "assembly" to mean the same thing - installing the tire on the wheel, not installing the wheel onto the hub. Let me reiterate, the reason for those letters is to describe the contours and they should allow for ease of mounting the tire onto the wheel.
As a side note: Those letters have nothing-what-so-ever to do with how the wheel mounts onto the vehicle and everything-to-do with how the tire mounts onto the wheel.
And, again, strength is independent of the letters. The exception to that would be if a wheel manufacturer were to produce the same wheel in both configurations and I don't think anyone does that.