Help ID pith helmet fash, South Africa?

[Westernhighlander]

Hello,
I would welcome any help identifying two helmets with the same flash sewn to side of puggaree/pagri. Both arepith/Wolseley helmets one is dated 1918 while the other is dated 1941/42.
The device is a diamond. Top is yellow, bottom black.
Based on info generously posted by another forum member (Madziro) on another thread (RWAFF) on this site, it appears that the flash may be that of either
A) 28 bn KAR,
B) Nyasaland Transport Coy or
C. Nyasaland Defence Force, (though have found elsewhere that the Nyasaland Defence Force was orange, not yellow)

Additionally, I am not sure if the Southern Rhodesia Transport Coy or 28KAR was around in 1918, so could the 1918 dated helmet may be the Southern Rhodesia Transport Coy and the 1941 dated helmet the 28KAR or were there other regiments using this device .... those are my thoughts, but would like to hear what the forum members think.

The three tables of southern African flashes were provided by and posted by forum memeber Madziro elsewhere on this site.many thanks.