Hi, the eight digit sequence on the back is the movement number and case code - 9029-6030, which you can use to order parts.
Unless somebody has one and knows the Model number, you'll likely have to look through the many catalogues here on the forum http://www.thewatchsite.com/13-ads-catalogs-vintage-current/
51 from the serial number gives January and probably 1985 or 1995 or 2005. It's only possible to know the year within a ten year slot.
Unfortunately the catalogues are usually region specific - and not all watches are sold in all regions. Usually the 8 digit code is what's used, the model name only appears in catalogues and shop windows.
I've found it's grey sibling here http://www.ebay.de/itm/SEIKO-ST-STEEL-SGB-BEZEL-9029-6040-Armbanduhr-/201077474451 which has a Seiko model number of SPU036J So the chances are that it's an SPU something.