Isthmus posted this over at WUS a while back:
http://forums.watchuseek.com/showthread.php?t=229254 Seiko 5 is Seiko's introductory line of automatic watches. It is also Seiko's oldest and longest continually used sub-brand. The "5" stands for the watch's 5 principal attributes:
1. Diaflex (unbreakable mainspring),
2. Diashock (Seiko's shock resistant design, equivalent to the Swiss "Incabloc"),
3. Automatic winding,
4. day/date indication, and
5. Water resistant.
The 5 logo has been applied to a wide variety of calibers and styles and is one of the few sub-brands still in use today. As an aside, there are some early 1980's intro level quartz models that attempted to capitalize on the recognition of the 5 logo and were branded as Seiko 5's, even though they did not meet the definition of the 5 logo. Apparently this did not work well and was not used for very long. Finding vintage seiko 5 quartz watches is generally uncommon.
There is no site run by seiko that identifies or is dedicated to all of their seiko 5 branded models. Your best bet for getting an idea of what is currently available is to look at local catalogs for markets where such watches are intended (mainly south east asia and parts of latin america). you can also look at the websites of online sellers who deal in Seiko 5's such as Roachman, PMWC, and Boris Affordable Wrist watches (AKA seiko5ers). They will all have a fairly extensive collection of current production seiko 5 models.
It is important to note that modern Seiko 5's all use 7s calibers. However the same is not true of vintage models. Over the decades the seiko 5 sub-brand has been offered in a very wide array of calibers, often at the same time. Also there are many cases of vintage models in which the same watch was offered both as a seiko 5 and as a regular seiko, with little or no other physical difference (sometimes they were branded in combination with other sub-brands, such as Sports, advan, speedtimer, DX, etc.)
Generally modern Seiko 5's are solidly built watches but not built to meet any ISO standards for professional usage. The only exceptions that I know of are the new Frankentmonster which was built to ISO diver standards and the now out of production 40th anniversary divers.
____________________________________
Later, Gary
Remember we are the caretakers of mechanical art.....
Member NAWCC, National Association of Watch & Clock Collectors
Subject: What does the "5" mean on Seiko 
Thu Feb 11, 2010 11:49 am
Never knew the meaning behind the whole "5" theme 