Here's the thing: every deck of Kem branded cards made from 1935 until 2004 when they got absorbed by USPCC has the same Ace of Spades. It looks like this one:
Notice the date of the copyright - ©1935. In most cases the Ace of Spades contains a 3- or 4-digit date code in the MMYY (or MYY) format for Month and Year. This AoS from a 'newer' upright case is from January, 1954. Sometimes that code is under the left lobe of the spade, sometimes the right.
Sometimes you'll see them advertised as 1947. Again, simple explanation. Check the replacement card put in every Kem deck made from 1947 to 1970's:
Sure enough - every single deck or pair at least has a card with "COPYRIGHT 1947" on it.
One last thing. You see an awful lot of claims that the case is Bakelite. Bakelite itself is rare, collectable and expensive. Do they all come in Bakelite cases? Of course not. Again, here's the visual cues:
The spread out K E M wide spaced letters IS indeed Bakelite! The small KEM centered on the box is not, so the two on the left are Bakelite and the one on the right is not.
Later cases came in more simple and plain black flat plastic cases, and even later in slip cases similar to Congress brand decks. Those were used after USPCC bought the company, but before 2004 when it was merged into Congress (more or less).
I'll take a shot in the dark and say that Bakelite was used from 1935 to 1950'ish, similar plastic cases in the same shape until about 1965, the simpler black plastic flat cases until about 1980 and the slip cases until 2004. Decks from this last period are "few and far between". Are those dates perfect? Certainly not! Are they close? You bet!
Last thing: how much are they worth? It varies too widely to say for sure, but Bakelite is the first publicly available plastic, a phenolic brittle plastic, which is why they are chipped or broken easily. The case itself is worth $25-$30. The card decks? Hard to say - in general they made a lot of them, not always, but most of the time. Some aren't worth much, some are worth $50-$100 and a few odd examples are worth more, up to $250, but that is truly unusual - most are in that $50-$75 range if they are in the old Bakelite case. Newer ones, generally less.