Maybe Zippy can tell by the castings which ones they are.
I've been away from the MP world for several years now, but I can remember the M1 suffix was the nicer, thicker/heavier Dalton casting that for a time came without pushrod holes per SS racers request. MP retooled the head at a huge cost but didn't really tell anyone except racers at the time. Being a humble "Tech guy" I found out about it months after the fact. I was told it had extra material for porting but was never told exactly how much extra.
To my understanding the Dalton castings come from those super expensive production style steel tools and as a result they're some of the more consistent parts out there.
The prior generation came from tooling which was normally used for making prototype parts, and was not really intended for long term use. In my discussions with Engineers, the old tooling was often pointed to as the reason for issues with the parts.
Not to say newer castings are totally immune from the issues all cast parts can have, core shift and porosity can and does happen....but the point being better tooling gives consistently better parts and fewer issues. I don't think anyone would disagree the Dalton castings are pretty clean. Also not to say all the older ones have those issues- most are just fine but they do have less material and are just less consistent.
As far as I know, Super stock/the huge round or oval intake ports they use with brazing and tons of material removal pretty much drove the whole thing.