If this watch looks familiar, it should. It’s identical to the CA-901 [QW134] watch except the keypad is in shiny silver rather than in dark blue or black.
An advertisement from Popular Science (1982) suggested the CA-851 is all-stainless steel construction which would make sense, otherwise it might just as well have been another keypad/colour option for the CA-901.
Having owned a few of these, I’m not convinced it is stainless though, as I’ve seen the same pitting on the back of the watch as in the CA-901 and other metal-plated Casio watches of the time.
The silver keypad generally doesn’t hold up well either, with scratches being much more obvious on the silver finish than the blue/black anodised finish of the CA-901.
If you ever find one with an unblemished keypad, though, you’ll see it’s a nice-looking watch, and gives a more quality appearance than the polysulfone (plastic) models of the CA-85, CA-86 and CA-90.
According to Casio’s own documentation, the CA-851 was priced at a rather bargain USD49.95 on launch compared to the USD69.95 price of the CA-901. These pricing guides can be a bit sketchy, though, as the advertisement shows the CA-851 with a RRP of USD69.95.
Casio made a lot of these, and used versions can be found on eBay most of the time, along with NOS replacement crystals, keypads and buttons at times.
Price-wise, they’re about the same as the CA-901 these days and if you prefer it over other models with the QW-134 module, you can swap out the module with a different version if you find a broken one that’s in nice cosmetic condition.
the more significant difference between 901 and 851 is in the bracelet which the 851 more stylish
i’ve had 901 and from all the watches i’ve owned i miss it the most