The 42mm rubber SMP strap will fit on the 41mm case. You might have to fiddle about with the springbars but it will snap into place with a bit of coaxing. The fit is flush and looks normal from the top.
Keep in mind metal bracelet end links will not fit as they don't allow for the amount of wiggle needed.
The 42mm strap I have is from my blue 42mm SMP. It is authentic Omega.