Well, I just looked up the reference list and it does indeed have six didgits. This bike has been laying around in body shop for a long time. What year did the Chinese start reproducing them ?
That's no Taiwan frame. This is the third Phantom I've seen with an S serial number and yours is the second one with a mystery S number. One of the three had an S number that fell in the range on the list. I believe there was an error when the SN list was entered on the net so your frame very well could be a 1950 Black Phantom.
1949
11/01 ------------------ F303038 ---------------- F308306
11/02 ------------------ F308307 ---------------- F309647
11/03 ------------------ F309648 ---------------- S312650
11/04 ------------------ S312651 ---------------- S315855
11/07 ------------------ S315856 ---------------- S318300
11/08 ------------------ S318301 ---------------- S321357
11/10 ------------------ F321358 ---------------- F326178
11/11 ------------------ F326179 ---------------- F331736
11/14 ------------------ F331737 ---------------- F333674
11/15 ------------------ F331737 ---------------- F333674
11/16 ------------------ F336648 ---------------- F339451
11/17 ------------------ F339452 ---------------- F341662
11/18 ------------------ F341663 ---------------- F344772
11/23 ------------------ F344773 ---------------- F345341
11/28 ------------------ F345342 ---------------- F347192
11/29 ------------------ F347193 ---------------- F348329
11/30 ------------------ F348330 ---------------- F349001