The Casio GC-50 Straight Flush is not a common watch at all.
While the Straight Flush watch does appear for sale periodically on eBay, it’s usually the plastic/resin version – the GC-10W – which is also nice, but the silver and stainless of the GC-50 gives it an extra touch of class and somehow makes it less game-like.
Those are, incidentally, the only two models where Casio made a poker game, and used the  module.
While a manual does help, you can work out the game without it – like anything it just makes it easier to understand.
There are three levels to the poker game (MACAO, LAS VEGAS, MONTE CARLO) which have 10 rounds each, so a full game is 30 hands.
The minimum bid (and ante) is $10, and max is $300.
You get dealt your cards first, then the computer player, and you see how many they choose to draw. You then keep or drop each of your cards and get replacements (standard 5-card draw).
The computer makes the first bet and you have the option to raise, call or fold. You’ll always lose your ante if you fold straight away, but that’s all.
As long as you have more money than the dealer at the end of each level, you’ll automatically progress to the next. If not, it’s game over. Likewise if you run out of money it’s over, and if you make it past the last round of the Monte Carlo level, same deal.
The watch remembers where you were up to if you don’t play a full game all at once. If you come in halfway through someone else’s game or don’t want to play it to conclusion, you can reset the game to the start by pressing the top left Adjust button (undocumented).
Other functions are a daily alarm and hourly time signal, and the usual time/date functions. There’s no light but there is an interesting stopwatch.
Interesting because it gives a nod to slot machines while it’s running with numbers spinning in three of the five spaces at the top of the screen (where the cards are in the poker game). Interesting too because there is no lap/split function. You get start/stop/reset. The other unusual thing as far as a Casio stopwatch goes, is that it only records to 1/10 of a second (almost all others record 1/100 second and a few 1/1000 second).
This particular watch came via the USA but I suspect it had made its way there from somewhere in Asia. The display looked brown, rather than black, which is not unusual for watches from Asian countries where they sometimes spend years in the direct sunlight in a shop window. A new polarising filter quickly fixed this though.
The other giveaway about it possibly having an Asian origin was the pitting on the back of the watch. Due to the climate and humidity, sweat from the wrist, over time, causes mild corrosion of the case (the non-stainless steel parts). In this case it’s not significant (although it is noticeable) – some watches, the Casio CFX-200 for example) often have severely corroded casebacks due to this.