I think the idea may have been adapted from the FN-FAL dust-cover mounted scope, which was a side-mount at about 30 degrees to the left from top. This allowed use of both the scope or the iron sights. I purchased the FN-FAL, converted to semi-automatic by inclusion of some Aust SLR parts, from a gunsmith/dealer in Box Hill Victoria. The weapon was disposed from the Dept of Defence (allegedly the Army Design Establishment) where it had been a test/trials rifle -it still had various tags and markings on it. Gordon McDonald had purchased it amongst a batch of other firearms, and I was lucky enough to walk in when he unpacked it, had the $$, and purchased it. All legal back then. It was a great rifle to use, and I never noted any problem with maintaining zero at ranges out to a few hundred metres. I had two dust covers, and would only place the heavier, scoped original cover in place when I wanted to actually shoot with the scope. I really enjoyed using it: lovely rifle.
Sadly, it's 'gone to God' now that such nasty, nasty things are illegal in Australia. (I replaced it with an M1A ....)
Mike