I just checked the fleet for 19-28FEB on the following routes.
HKG-SFO 872
34C except: 343 on 19FEB, 34S on 22FEB
HKG-LHR 255
74B
HKG-LHR 251
74B
except: 74J on 24FEB, 744 on 25FEB, 27FEB
HKG-SYD 101
333
HKG-SYD 111
74B except: 333 on 19FEB,23FEB
HKG-YYZ 828
34C except: 343 on 19FEB, 21FEB, 34S on 20FEB
HKG-CDG 261
74B
so there's no way to distinguish between 747 with the new J and one with the old J, if both are given 74B?
I also came across 343 on SFO-HKG in early Jan., which had the old J and the old F. I wonder if this has already been revamped.