When looking for the vintage SEIKO 6105-8110 or 6105-8119 automatic diver, an important thing to check is the crown. Some 6105-8110/8119 in the market have fake crowns. The water resistant quality of the fake crowns is unknown. Its internal geometry is also different. Most of the fake SEIKO 6105-8110/8119 crowns requires cutting off the tip of the original crown tube. So if you see a 6105-8110/8119 with fake crown, most probably the crown tube has been cut off or modified and the case becomes worthless piece of junk.
A simple way to spot fake SEIKO 6105-8110/8119 crowns is by counting number of crown "teeth" per section. The original 6105-8110/8119 crown has six groups of teeth. Each group has seven to eight teeth. While the fake one has only five teeth per group. See pictures below to see comparison between genuine and fake SEIKO 6105-8110/8119 crown.
FAKE SEIKO 6105-8110/8119 crown having five teeth per group.
Happy hunting and caveat emptor!
UDPATE 2014-05-04: SEIKO 6105-811x crown with 6 ribs per group
Lately I saw some original 6105-8110 and 6105-8119 crown with 6 teeth per group. Probably this is from early production batches of 6105-811x (1970 to mid 1970s). Below are two pictures submitted by Kevin a member of watchuseek.com forum which I have enlarged and edited for clarity. The watch is a SEIKO 6105-8119 from April 1973. The crown has 6 ribs per group.