I had a few guns Mag-Na-Ported back in the day as did many when the concept was introduced and advertised by Larry Kelly. It was not a S&W option but years later the factory did put out some models with the Mag-Na-Port but your's is not one of them.
I had a 629 no dash the Performance Center did. Well actually I shipped the gun to S&W PC and they in turn shipped it to Mag-na-port. It looked just as yours with the same shaped ports cut in the same location.
The only thing the dealer told me about the gun was he received it in trade on a New S&W 500 and an Aimpoint scope set-up.
I haven't had a chance to get to the range with it yet (or quite a few others! I hate February.) I'm planning on shooting my light and mid range handloads out of this one and the heavy stuff through my 629-6 or Redhawk.