According to my Gould, Third edition 2000, set 201 was originally formed BTK 3730-CK 5635-BTK 3731. The BTKs were part of order 488, which comprised TO 1398-1400, 3rd Brakes 3728-3731 and 6-compartment 3733, CK 5635, BCK 6674, FK 7676 and Dining Saloon 7958 (my emphasis).
The distinction between 3728-3731 and 3733 implies that 3728-3731 were 4-compartment brakes, and the placing of set 201 in the same workings as low-window 3-sets 390-399/445-448, which also had 4-compartment brakes, seals the deal.
King also gives 3730/3731 as 4-compartment brakes...