Tange 900 was a seamed version of Tange
#2 . Using seamed tubes brought butted tubing down to a lower price level. A seamed, butted CrMo main triangle with lesser grade CMn stays and forks was a practice that started in the early 1980s on lower mid-range frames. The Tange 900 & Mangaloy combination was common in the mid-1980s.
The frankenbike nature is likely due due to the policy of robbing components from one one bicycle to repair another for a customer. This was was fairly common in the retail chains which did not have an in-house bicycle shop and did not stock repair parts. It was quite common to see warehouses with a row bicycles that had been partially stripped of components for this purpose. Every few years they would order parts to rebuild the bicycles and dispose of them on sale.
I've just noticed that both quick release levers for the wheels appear to be improperly adjusted. The rear does not appear to be in the fully locked position while the front appears to have been fastened by tightening the nut and not using the cam action of the lever, as the lever is pointed in the wrong direction.
Did you get the serial number? I'd appreciate it, so that I can add another brand to my serial number guide. TIA.