Yes it is legit and is a nice looking one. By the time your Seiko rolled out, Seiko realized the gold medallions were prone to being eaten away quickly by sweat and went to a regular case back.
The crystal looks good. Those were mineral glass and prone to chipping on the beveled edges.
Make sure you have them show you the date is working as a lot of the 5xxx models (KS, LM etc) suffered from the date changer cracking and causing the quick set feature not to work. Even if the date does work, go slow in adjusting the time.
Remember no matter how well the watch is working, have it serviced before using it as those hibeats will run well being starved of oil but will chew itself up in the process and KS parts are not exactly easy to find for cheap
