I don't know the answer to this question.
I hope T Woodroffe will answer, since he almost certainly knows.
There is the locomotive diagram which provides a simplified but generally accurate idea of the shape of the casing.
There is a complete (if worn) streamlined tender at Seymour.
The casing was pretty simple, except for the nose doors.
There were several distinct components.
The "skyline" casing which covered the stack and the dome and safety valves.
The two large side valances that were the majority of the "streamlining".
The two nose doors that covered the smokebox front.
The lower front casing that faired the nose doors into the buffer beam, coupler and pilot.
The pilot itself changed from solid to a curved version of the steel bar pilot originally fitted.
The side valances were originally faired into the pilot but were cut back to the buffer beam in the 1940s.
(The recent Trainbuilder model represents the cut back version)
It is possible that there were no detail drawings prepared for the S class streamlining. Certainly the first Gresley A4 was built up from basic sketches and drawings for later locomotives were prepared based on the work done on the first locomotive. In fact the first four had an incorrect radius on the front "wedge" that resulted in the casing being too long at the buffer beam, corrected later.
The S class streamlining was much simpler than the A4, being largely flat surfaces added to the existing locomotive.
One S class was damaged in a level crossing accident about 1951 and the nose doors and fairing were removed and repaired or replaced. Photos of the loco running with the front of the casing removed showed how little the basic locomotive was changed, since it ran with the original buffer beam exposed and a flat pressed steel pilot attached. Beyond the side valances, only short side plates beside the smokebox faired the nose doors into the existing boiler.