I'm also not aware of any custom breech for the QB78 family, "square" or otherwise.
There is one basic type of factory breech, as fitted to the QB 78, QB78 Deluxe and QB79. This is factory modified by two holes tapped and drilled for a flat sight rail on the AR2078/A and AR2079 models. There is another version for the incredibly-rare AR2078B and 2079B, side-lever cocking models, but again this is just a version of the standard breech. These factory breech assemblies are pretty "bombproof" and reliable - in my experience - that's probably the reason for no after-market versions.
Hope this helps...
Stephen Archer
Archer Airguns Inc.
http://www.archerairuns.com