1939 Jefferson Nickel Sans Serif Variety?

I know very little to nothing about nickels, so my help here should be taken as a guess based on general die striking dynamics...

These two coins appear to be struck using different deisgns - obviously why you posted them. The one thing that bugs me a little about it is that one of them is a less-worn earlier die state coin, and the other is a more worn later die state example. This "could" have something to do with the fatter letters and missing serifs.

One thing I do know, however, is that there were a number of design modifications that affect the earlier Jefferson nickels and you could have rediscovered a marker to tell the difference, or you could well have discovered something new there.

My suggestion - keep the two out in their own separate place and keep looking. It's the only way you'll know the following:

1. Are they really different designs?

2. Which one is really more scarce if they are different designs?

I have had a similar "discovery" with 1909 Lincoln cents. There are actually two completely separate reverses used, and I'm not talking about the VDB initials. ALL of the letters on the reverse have two distinct different shapes.

Note that we are discussing the same topic here and they are definitely related...they both belong to the first couple of years of a given design's use. I'm sure there are a number of other cases where detectable modifications were made within the first couple of years of the use of a lot of our designs. I know the obverse of the Mercury dime was changed...others?

Well, the terms "stage" and "state" are completely different terms and cannot be used interchangeably. You are intending on saying "late die state" - not "late die stage."

A "stage" is a point on a single given die where something happens to add or take away a marker used to identify that die. A die crack develops, and that die changes from one stage to the next.

A "state" refers only to the age of a die at the point that it struck a specific coin. This can be determined by looking at the amount of wear on the die as evidenced by missing detail and flow lines.

In this particular case, I doublt that it was die wear alone that would cause a honking HUGE serif on the bottom of the N of UNUM to become only a hint of what could be a serif. I believe Numismo is onto somethig here, but further down in my post I explained that there were indeed different designs used in that era and this might be a sign of just that.

Just my opinion, but they both seem to be the same design.
Looking at all the angles etc. The coins appear to be just different die states. Second one getting mushy and worn. If you look carefully
at the spacings, width and height etc. you can see the similarities.

At first glance I thought varying die states and circulation wear, but I would keep searching through 39s and see if you can document the differences with repeated examples.