All Technikas since the first III of 1946 use the same size interchangeable back. A brand new 2020 Master technika back will fit fine on a 1946 Technika III camera. Yours looks the same but I have no proof of this being the case so I cannot confirm it is. If you get a later back let us know if it fits.
The 9x12 back you show above is from the earliest technika III model and is only for 9x12 sized thin plate holders which can hold 9x12 film sheathes and take 9x12 sheet film. It is too small for 4x5 film. Next Linhof made a spring back and eventually a graflex/international style back which had 3 versions to get to what is on the technikas of today...
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