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Off the bat, I'll admit to knowing precious little about what held it up, other than "interwar economic issues" for Britain.
Therefore, I pose two questions:
1) What could have changed to push foward its completion (Muted CP victory, albeit with Tanzania falling de-facto to the Brits? = Stronger impetus to secure the region?)
...and
2) What would have been the consequences of its completion (call it finished in... the mid/late 20's / Pre Great Depression) on the development / connection with the British holdings in Southern Africa and the rest of Africa?