The DIN Plug on the back is for the Wired Remote for the AUTO SCAN. Optional on the 500-TX and included with the 800-T. And they both use the same manuals.
Good possibility the faceplate was swapped at some point. The 500-TX and 800-T are for all intents and purposes the same unit except for the faceplate coloring, the single voltage trannie(500-TX) vs. the Multi voltage trannie on the 800-T. There are some minor wiring differences (mainly fusing, and such) but otherwise identical. You could have the faceplate painted to match the 800-T colors and then lettered. The 800-T would have been sold overseas at PX's by FISHER like the 1800. As it came out in mid '68 for the '69 Model year(Sept 68) and lasted until the end of the '71 model year (Aug71) I'm thinking that it either replaced the 1800 or was sold side by side with the 1800 until supplies of the 1800 petered out.