The first hints we get
When perusing the PC software, Upload VR says that it found a setting in the user interface code that allows PC users to select between 50Hz and 60Hz room lighting along with an option for auto-selecting between the two. The description for the setting read "lighting frequency for Rift S cameras to adjust to room lighting."
Another setting references cameras in a section for the Rift headset, which is interesting because the Oculus PC software recognizes USB-powered sensors as separate devices. The fact that cameras are included in that section suggests that they are on-board the headset. A software-based IPD adjustment setting was also found that specifically references the Rift S by name.
The description for that setting read, "Set your IPD (Interpupillary Distance), the distance between the centers of your eyes, to optimize the scale of objects in VR. Description for the IPD adjustment setting where users can set their interpupillary distance as respected by Rift S."
The inclusion of the setting makes it appear that there will be no hardware IPD adjustment for the Rift S hinting that the lenses inside the headset might be in a fixed position. The Oculus Quest device mentioned previously is expected to include a physical IPD adjustment. Speculation suggests that one reason for no physical IPD adjustment could be that the Rift S will use a single LCD panel similar to what is used inside the Oculus Go. No official comment on the Oculus S has been made at this time.
Rumors have suggested that the Oculus Quest will also launch this year.