1941-S Is This MD Or Possibly RPM?

Looks to be MD. You can also see it above the date in the same direction.

Yep. Machine Doubling. Almost looks like stairs with the MD. When they are the same on the mintmark and the date on the pre 1990 coins, it is Machine Doubling. On the post 1990 coins, it is the opposite, it is a doubled die. But Machine Doubling can also affect doubled dies as well:
Note the dates on the Real DDO.

But then look at the yellow arrows. That is showing the Machine Doubling on the DDO. But the 1968-1972 cents have the most Machine Doubling during those years. So when the see the same event happening on the pre 1990 coins with the date and the mintmark in the same direction, it is Machine Doubling. Why is that? Because the die creation is one step of the hub process, The adding the mintmark by hand is a separate process. To have a DDO and RPM does happen. But not in the same direction as the Machine Doubling. That is what you are seeing. Machine Doubling can happen on doubled dies, but a doubled die has the doubling on the die, thus the term Doubled Die.