The whole Global vs Japan only is a bit confusing since the 7506 was also sold in Japan with the CD900ST. I think an important fact is that the CD900ST and M1ST are sold and supported by Sony Music Solutions which essentially deals with all the physical stuff from packaging to studio management for Sony Music Japan.
On the other hand, the 7506 and M1 are both sold by Sony Electronics. The MV1 is also available in Japan but sold by Sony Electronics, not Sony Music Solutions.
It could just be that the M1 is made by Sony Music Solutions for Sony Electronics, similar to how the 7550 is the Sony Electronics version of the EX800ST. Both of those were sold in Japan but only the 7550 was sold abroad.
As for sounding different? It could possibly be? But I'm less inclined to think that way.
There's also this weird roundabout way that both Sony's use a different Sony corporate entity, Sony/Taiyo Corporation to actually the make the products. For example the M1ST, Z1R and the Just ear IEMs (which is actually a different company from Sony Electronics) all are manufactured by Sony/Taiyo. So that's three different Sony's who manufacture their stuff at an entirely different Sony
Anyway, I think it will be a 7550 vs EX800ST sort of deal. Small differences in the replaceable parts because if the M1 were to use the pads of the M1ST, then you'd need to get replacement pads from Sony Music Solutions so instead Sony Electronics have their own pads. Same thing happened for ear tips for the EX800ST vs 7550 and the 7506 vs CD900ST even though they're perfectly swappable. Just Sony being weird.