To the best of my knowledge S&W has NEVER offered a fixed-sight .44 Magnum
Ahem...S&W made 300 2½" 629's with fixed sights for Camfour back in 2000. You even responded to a thread four years ago that mentioned them.
Then there's the 3" Carry Comp for Lew Horton back in 1992.
However, if it's a .44Mag and marked model 58, it's most certainly a custom conversion.