Take a look at http://speedmaster101.com/. The image on the homepage looks very similar to your dial.
Used speedy 101 to compare in the beginning. Still unsure. " s" looks elongated and p widened. Not my watch but one I was looking to buy
This dial is domed, not stepped and I agree should be on a Speedmaster built after mid 1970's. I am not certain of the exact date it changed. I have seen them on -76 onwards