slanted end goes outward towards you, the installer.
at least that's what i've decided. otherwise, the other end of the key which is square cut to the other surfaces of the key may not, or possibly will not, present a properly square surface to the large washer and nut on the end of a tapered axle.
BTW, the last pic shows, on the right hand side, one of the two (the vertically mounted one) handbrake levers. on my backing plate (a '41) there are two clips attached to the backing plate on the right hand side. they are stacked one on top of the other, one to secure the brake shoe and an additional one to secure the handbrake lever. i see that your backing plate has only the one to secure the shoe. not sure that it makes a difference though.

but then again, your backing plate looks new and mine are probably 85 years old. sometimes re-manufacturers simplify things a bit that prove over time to be unnecessary, i think.