The 15MP Sony IMX521CQR sensor was first leaked several months ago. Now since Sony a7S III is rumored to be announced soon, it is very likely that Sony a7S III will use this 15MP IMX521CQR sensor, you can take a look at full specs sheet of IMX521CQR below. Sony a7S III will shoot 4K60p video, and a possible of 61MP mode.
The upcoming Sony a7S III is expected to be announced in the second half of 2019. According to previous interview with Sony manager, upcoming Sony a7S III may feature with 10bit 4:2:2 and 4K video at 60p. And a trusted source also told us that Sony a7S III could feature with a new 15MP full frame sensor, while current a7S II has a 12.2MP full frame sensor.
A new series of Sony full frame CMOS sensors now leaked on the web. These image sensors are all full frame sensors: IMX311, IMX313, IMX409, IMX521, IMX554.