A few reasons:
1) They replaced the original Seamaster dial and hands with the ones you have now;
2) They lost the original caseback and put that one on; OR
3) Something that
@tyrantlizardrex suggested to me in a thread I posted earlier - your watch is part of the 'International Collection' and may just simply not have the Seamaster name on the dial. Some sleuthing on the Omega website turned up this link based on your caseback number:
http://www.omegawatches.com/planet-omega/heritage/vintage-details/15142/
Having a look at it, notice that all the specs match your watch, including the gold-hour markers and index hands, a fully stainless steel case, a 24 jewel movement that is probably a Cal. 552, and a central sweep hand.
The watch is a beauty! I can see that the dial is most likely original and aging well; you also have a signed crown, caseback, movement, and crystal (you can see a tiny Omega logo in your last photo on the glass.) The seconds hand seems a bit short; however, I'm not an expert in recognising what the correct one would be. It does appear age appropriate though.
To get the most out of your watch I would recommended a service straight away before you wear it on a daily basis. If you post up your location I'm sure people in the same area, town, state or country will be happy to let you know where to take it. They will know reputable watchmakers who are experienced and have handled plenty of vintage Omegas!
Hope this helps
EDIT: This would appear to be the auction link:
http://www.ebay.com/itm/Vintage-Ome...o-mw-Men-s-SS-Watch-14700-SC-61-/222213987491Purchases made through these links may earn this site a commission from the eBay Partner Network
Fellow Aussie?
@JimInOz can point you to a watchmaker [or do it himself I believe
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