Yes, the Model 29 is built on the N frame and so are the Models 27 and 28. Both are large frame 357 Magnums. There are also 357 Mags built on K frames and even J frames, but if the guy said they were from a Model 28, then they were from an N frame. As noted, the Model 28 shipped with both Oversized Target grips and Magna grips, which are not as large.
There is another wrinkle. Originally, S&W grips were custom fit to the gun. Before hardening, the grips and the frame would be sanded down together so the grips fit the gun perfectly. Then the frame would be sent to another department to be hardened and blued while the grips were sent to a different department to be varnished. The grips would be reunited with their specific frame at final assembly. Since the grinding process to mate up the grips with the frame was a hand process there could be variation from gun to gun, and grips from one frame may not fit another frame of the same size perfectly. There can be some variation when they are put on another gun. They often do not mate up perfectly line to line with another gun.
This was all pre-MIM parts and pre-locks. I have no idea how Smith is currently fitting grips to their frames, but I'll bet CNC equipment is used.