The L2D "Tabby" was an independent Japanese cargo development of the DC-3 (they bought several before the war) rather than a simple C-47 knock-off. It was the same length as a standard Gooney, but had a reconfigured nav/radio compartment with extra windows. The cargo door was also slightly different in shape and location than the C-47, and of course it had Japanese engines and props.
The version with the upper turret was the L2D3. It wasn't actually a power turret, but a cupola with a hand-held gun, similar to the early variants of the "Betty" bomber. It was a late-war prototype, and I don't think more than a handful were built (if that many.)
I did a fair bit of research on the L2D a couple of years ago for a model project. I converted an Italeri 1/72 C-47 into an L2D2. I could have really used a couple of those pictures then..why is it the references you need for a project always seem to magically turn up after it's finished?
Anyway, here's the result...
SN