Number 75 is a different story - for RHD they had to come up with a different solution, so this is the bracket that is attached to chassis.
You are LHD, so we are talking bracket no. 74 - for the clutch fluid resrvoir. On Mercedes part list it is marked M (for Manual). I think then that you are correct in your conclusion.
Then why I have seen it on some automatic cars, including mine? I do not know
. Someone fitted it over the last 50 years? Because this bracket was also used for additional fuse box? No idea. Not everything there is 100% regular and easy to explain...
My unsupported or almost unsupported theory is: Mercedes used this bracket in the cad plated nice version on both automatic and manual cars up to a certain moment. For automatic cars they used it for washer reservoir support. Then they changed the bracket to a cheaper one - black painted (there is another part number for it). Used it for automatic for some time to make it more "elegant" for the washer bottle to hang on something. Then they decided: ok, if we do not have the clutch fluid tank, we can delete this bracket because we can just stick this washer bottle and it will hold good enough. We will save a couple of DM.
But it may be wrong. I would have to have some statistics on older cars M or A with or without bracket, assuming they are in original state.