Note that late 2015/early 2016 sees the reissuance of the cushion case via the SRP773/775/777/779, so in a sense what we are seeing is Seiko's design aesthetic going full circle.
The 6309 line is powered by a 17 jewel automatic movement with day/date which neither hacks or handwinds. Power reserve is around 45 hours fully wound.
Case diameter is 44mm excluding the crown, lug-to-lug length is 46mm. Case thickness is 14mm, and lug width is 22mm. In an era when the typical men's watch was less than 40mm in diameter, this was considered huge at the time. 44mm nowadays is pretty much par for the course.
Water resistance is 150m, 50m short of modern day dive watch standards. I wouldn't recommend diving or even swimming with one now, unless the seals have been renewed and the watch pressure tested.
This particular sample in my collection is all original despite being more than thirty years old (date of manufacture per serial number is July 1984). The only replacement component is the relatively current Seiko Jubilee-style stainless steel bracelet, which is to be expected since this watch was originally supplied with a rubber strap, which has long gone.