BTW: Supermarine was given an 'S' makers code by the US Navy (along with Sikorsky/Vought-Sikorsky) so I assumed that Short would be 'S' as well.
The same code was only assigned in cases where the different manufacturers' types didn't overlap,
as people weren't likely to confuse a Stearman trainer (N2S) with a Sikorsky flying boat (PBS) with a
Supermarine fighter (FS).
So, assigning 'S' for Shorts would be unlikely, and even if they did, a PB2S would have to be a second
Patrol Bomber from Sikorsky, not a first of type from Shorts.
In 1922 Blackburn, De Havilland, Handley-Page, Parnall and Vickers had 'two-letter' codes assigned,
respectively BS, DH, HP, PL and VK, so I suppose a revival of that scheme could generate a code of SB
for Short Brothers.
In that case a Sunderland I, II or III for the USN could be a PBSB-1, 2, 3; and a Sunderland V a PB2SB-1.
p.s. so while thinking about hypothetical two-letter codes how about the rest of the Brit manufacturers:
AS = Airspeed
AW = Armstrong Whitworth
AV = Avro
BR = Bristol
BP = Boulton-Paul
EE = English Electric
FY = Fairey
GL = Gloster
HK = Hawker
MA = Miles
SA = Saunders Roe/SARO
WL = Westland