Thanks Don 
That's interesting, I hadn't thought of the deck seal as a replacement for the tax stamp or maker's mark... I like those old (and some new) stamps Some have fascinating stories to tell - and then there's that sense of connection to the hand that made the article (ceramic, silver, paper, leather...). I've seen many decks with the stamp on Ace of Hearts or Spades, but never on 4 of Hearts of Diamonds - I'm sure there's a story or reason behind why they chose that card
There's been a few cases in history, particularly in the early days of card making in Europe, when cards were often made by hand, where you might find a maker's mark made somewhere on the deck, in much the same way that an artist signs a painting.
The cards we know today as the International Standard originated in Rouen, France. The Parisian design was more popular with the French populace - the Rouen design was made largely for export, and much of those exports went to England, as Rouen is on the coast of the English Channel.
Elaborate Aces of Spades started in England, I think. The British started copying the decks that were made in Rouen, making gradual alterations to the design over time, including the fancy Ace of Spades - they functioned as identification of the manufacturer as well as proof that the duty (tax) was paid. Old English decks actually say right on the Ace of Spades how much duty was paid (I've seen one marked "two pence").
In the US, the taxes paid on cards were proven in the form of stamps purchased from the government - but we kept the tradition from the Brits of having elaborate Aces of Spades. The stamps were originally made by the Internal Revenue Service, though in time they gave permission to individual manufacturers to make their own stamps - as long as the taxes were paid. Additionally, some states also charged taxes and used stamps to prove they were paid - Alabama is the last state in the country to continue with a separate tax beyond "sales tax" specifically for playing cards, and they issue state revenue tax stamps.
In 1965, the tax on playing cards was repealed, but by this time, people had become accustomed over several decades to seeing tax stamps on their decks - before cellophane, they kept the deck sealed shut, insuring your deck was brand new and hadn't been tampered with. Most manufacturers still in operation at the time opted to continue using a deck seal in the form of a company stamp (in essence, it was a type of maker's mark, used to insure the product was brand new). The moisture-activated adhesive stamps eventually gave way to stickers - and in some cases, no seal at all.
In the early days of the current wave of modern custom deck collecting, most companies making new card designs got the standard deck seals offered by the manufacturer. After a handful of years, the trend changed to be a deck without a seal - people often peeled the sticker off anyway, feeling that it marred the great tuck box art. This trend nearly completely died when manufacturers started offering custom-designed seals for those willing to pay for them - today, that's how most of the most popular new custom decks get packaged.
As far as the tax stamp being on the 4 of Diamonds in Spain... I guess the 4 of Diamonds was chosen because it was less conspicuous. There's also the fact that often the decks found in the International Standard design were imported (remember, Spain had its own traditional deck design, a 48 card non-indexed deck still printed and played with to this day). That meant the Ace of Spades was already "spoken for," as it were, so a different card had to be chosen for the import duty. If you check out World of Playing Cards (wopc.co.uk) you can find some examples of this - I remember a photo of an imported US-made USPC deck sold in Argentina, displayed with a tax stamp on the 4 of Diamonds from the Argentinian government proving the duty was paid. Imagine being the poor tax clerk at the Customs Office having to crack open every single imported deck, digging out the 4 of Diamonds, slapping a rubber stamp on it, then returning the cards and resealing the pack...drudgery...
