I have restored many cases for collectors and during that endeavor I have found two cases with corner joints other than the finger jointed!
These pictures are of the lap type joint I have seen. Both cases had the 1979-1983 type liners(small tool compartment).
dnonac, the picture you posted has 45 deg corners which is common on more modern cases generally used with special guns. Also you will notice the wood is thicker and the latch is installed with small screws. All the Mahogany cases of the 1960-1983 period used thin wood which required they use a rivet type fasterner for the hardware. As you stated the liners fit perfectly so the case you have could have been an experimental case that never made it to production.
jcelect SWCA#LM723