I found a matching circuit for the Hitachi KM-1520ZC in the 1520H. Apparently it was the same radio with a different faceplate for cars from Detroit. Scanned and uploaded it in hopes it can be of help for someone else. As far as I've been able to find, no known digitized technical manuals exist for this stereo until now!
https://objects-us-east-1.dream.io/dox/KM-1520H.pdf
With the help of a Datsun radio guru I was able to confirm at least most if not all of the circuitry matches, and I used it to locate a broken trace in my own radio. Some part numbers match replacements on Digikey/Mouser, and it contains charts with specs for just about every component in the radio. I will be replacing my electrolytic capacitors using the information within.
I also plan to create a remotely mounted switched auxillary port to the dashboard, if anyone is interested in following along with that project. I'll be using a vintage-looking 5-pin DIN connector spliced into the radio chassis to facilitate this and 3d printed parts at the plug end to nicely integrate with the dash.