No, I'm struggling, but there is no way I'm going to give up.
In the mean time, I've found info on other units and users of armoured cars of various types including MH/SARC Mk.IIIs. From:
Federated Malay States Volunteer Force
"FMSVF Armoured Car Coy, Marman Harringtons and Lanchesters. It expanded to form a Regiment on 24 December 1941, of 2 Squadrons. Uniform: Green berets with red "X" (Ken Archer, W.T. Dunne, J.L. Kennedy, D.H. Telfer; F.J. Woodward. Records, interviews, letters."
Straits Settlement Volunteer Force
"SSVF ARMOURED CAR COMPANY. (39 ) (Disbanded 16.12.41 due to age of Cars. Personnel transferred to SSVF CARRIER PLATOON (Bren carriers)."
Note: On the same site is some information on armoured vehicles built locally during the early 30s which, at the outbreak of war were found to be unsuitable "due to age of cars".
Unfederated Malay States Volunteer Force (UFMSVF)
"THE KEDAH VOLUNTEERS (589 571) Kedah had its own unit of the Armoured Car Coy. Prior to war Unfederated States usually had a car stationed in each state. Kedah's Marman Harrington Armoured Cars were disbanded on 18 December 1941 and joined the body of FMSVF Cars."
If one follows the movements of the FMSVF Armoured Car Regiment's 2 squadrons formed on 24 December 1941, more is bound to surface.