Thanks for your replies guys.
This is great. I always felt it looked like a keyboard of some sort but was confused by it so was genuinely interested to see what experts thought.
I asked question somewhere else and got the reply ‘cos he’s the suicide king and mentally unstable people are often musically gifted’, which didn’t ring true seeing as we kind of know the suicide thing is more attributed to bad reproduction.
The King of Hearts wasn't always the "suicide King." Originally, he carried an axe, not a sword, but as you mentioned, when the Rouen design was imported from France to the UK, then copied by UK printers, then imported to the US from the UK, then copied by US printers (all over the course of the 18th and 19th centuries), it was a little like playing the game "Operator" in print form - something got altered along the way from handcrafted copy to handcrafted copy. It was in part accidental, because printing wasn't as high quality as it is today for cheaply mass-produced playing cards, especially when cards were hand-colored, and it was in part intentional, as many manufacturers made subtle tweaks to the design to make it look more like their own and less like something they copied from someone else.
On a related note, dictionary printers have done something similar over the years to prevent competitors from simply swiping their definitions and reprinting them under their own name. Dictionary makers will usually toss in an occasional, obscure-looking, completely INVENTED word that never existed in the language before, something really off-the-wall and very unlikely to ever be used in a conversation or looked up for any reason - it's sort of like their version of copy protection or watermarking. If that word started appearing in other publishers' dictionaries, and not simply because some dictionary-reading smartypants started actually using it in day-to-day conversation and it caught on, they could prove that those other printers plagiarized their work and violated their copyrights. These days, however, it's impossible to copyright or even trademark the common faces of a deck of playing cards, for much the same reason that one can't copyright the alphabet or patent the spoon - there's too much prior art in existence and it's been part of the public domain for too long. Other elements can be protected - for example, despite being too old to protect as copyrights, the USPC protects their unique jokers, Aces of Spades and card back designs of their most commonly made decks as trademarks, which can be renewed in perpetuity as long as they are adequately protected from dilution. (It's that "dilution" part of trademark law that forces USPC to prevent designers from making alterations to those design elements when printing their own decks based on those designs, hence the reason why you can no longer make a marked or otherwise gaffed version of a Bicycle Rider Back or a Bee Diamond Back. In the case of the Bee design, they're especially protective these days, reserving it almost exclusively for use by casinos rather than by custom deck designers.)
While the King of Hearts looks like he's stabbing himself in the head, in reality, he's holding his sword BEHIND his head, as if preparing to make an overhand strike (what a Japanese martial artist would call "shomen uchi", or, if he swung at an angle rather than straight overhead, "yokomen uchi").