That image looks like something that could workI think you’ve landed on the notch as an assembly aid - the photos look like they're undercut , so you could slide in the spacer, and it couldn't fall out while you are wrestling with the brake mounting hardware.
It adds a lot of complexity to the the spacer if you are machining them in small batches and if you can put up with having to hole the spacer while you mount the brake, you don't really need any protrusion at the bottom.
I really think you can get away with a single hole and only worry about the height dimension. - I don't think there is enough cross sectional area in anything that would pick up the other "main" mount- to do more than the properly tightened bolt does.
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If you had these front brakes mounted, could you ride it and test it or is that ways off?