Ted, this is great information! Yes, you are correct that 3rd Rail made the SP MT-4 and MT-5. The MT-4 was available in both black and the partial Daylight versions, while the MT-5 was only available in black. All models had the skyline casings but for some reason only the MT-5 received the graphite painted smoke box.
Here are the MT-4's. I'm still hunting for a MT-5.
I'm also including a photo of 3rd Rails B&O T-4a, which isn't seen often. I found mine out of pure luck.
Thanks for sharing the info!
Nick