Hi Will
I was looking at the 1923 dress regs with regard to the orders of dress when I responded to Brian's message, but know taking another look, the UDF 1923 regulations only mention a JACKET (ie. Service dress in drab with step collar)
and a FROCK (Modified jacket with stand collar in blue serge).
The blue FROCK was worn in Review Order B with sash and in Undress order with Sam Brown belt.
The sash in 1923 was a gold and crimson silk sash for General officers and blue and red silk for staff officers. Other officers dont appear to have worn a sash.
The two uniforms you have (Blue/Black and khaki) are in fact full dress TUNIC's and this appears to have been adopted c.1934 (or at least referenced to in the 1934 regulations)
The 1934 regulations only mention a gold and crimson net silk sash to be worn by all staff officers in Full Dress Review order and Modified Full Dress, so the blue sash appears to no longer be worn.
I was given a very scattered (missing many pages) photocopy of the 1934 regulations, so am unable to give precise details as to the orders of dress. The sash is mentioned on page 15 (item number 35) and the Full dress tunic on page 17 (item number 47). I dont have the pages of the officers dress orders, so cannot elaborate on the whether the Sam Brown was worn with the Tunic or not. With that I retract my statement regarding the Museums display of the khaki Tunic with Sam Brown
Regards
Steven