In the last week, I've found '49-S and '50-P nickels at work. Both of these coins have mintages of under 10 million which is low for the era and can be considered semi-key dates if not full on key dates. Uncommon coins are still out there, you just have to be patient. I've had good luck on 1960 and back nickels over the last few days. No war nickels for a few weeks and no buffalos or Vs but some decent '40s and '50s nickels.
I think it was last summer, I got a nearly dateless S-mint Buffalo nickel in change at a supermarket. But I was able to identify it as a Type I 1913-S because of the mound.
The mintage was a fraction of the mintage of other period nickels (except for the pesky 1950-D) so I consider it a semi-key.
Mintage is a very poor indicator of key or semi-key status. If you go by mintage then a 1933 Double Eagle is neither. Check the catalog value.
What would you consider a key date or semi-key date Jefferson nickel? Don't say just the 1950-D and the '38s and '39s because there have to be more.
Going on mintage alone...those coins are the lower minted coins in the series. However, outside of ultra high grade conditional rarities, the Jefferson Nickel set has no true (circulation strike) "key coins" IMHO. Every circulation strike issued in this set is common in MS.
Those are great finds. These events are what make this hobby enjoyable for many people, not always what the book says it's worth.
I feel like I have gone over this with you before, but there doesn't have to be more. The keys are the 1950-D, 1939-D, and the varieties. I would not even include the 38-S or 39-S as keys or semi-keys. Essentially, outside of the coins I listed, Jefferson Nickels are not considered collectible in the circulated grades. Heck, most people don't consider them collectible in the lower mint state grades. The Jefferson series is a gem+ series. Feel free to collect what you like and if finding old circulated Jeffersons makes you happy, then by all means keep them, but you are going to face resistance if you try to get others to agree that your finds have value.