I'd like to give a big F-U to eBay sellers who want $25 shipping for a magazine. Shipping for this beast from Ohio to the Democratic People's Republic of California was $18. That's the difference between a sale and having something collect dust and dead spiders in your storage room.
The 5700-series came out around 1966/67, and replaced the 5200-series. According to the Reel to Reel Tape Recorder Museum,the 5700s incorporated some mechanical improvements and headphone jacks.
The 5750 is the main unit—swring-open, detachable speakers that double as a cover; hardwood cabinet. My 5740 is the same thing except it has vinyl covering, which is probably better if you lug it around.
Also in the series is the 5730, which had the speakers built directly into the cabinet in the old portable style. The 5710 is a monoaural version of the 5730. The 5720 is a deck (which means it has no amplifier or speakers: you have to cable it up to something in order to hear it). The 5720 also lacks a headphone jack.
There was also a deluxe 5800, which was basically the same as the 5750 but with modular: separate speakers, and a separate AM/FM radio tuner.
The service docs can be found in Sams Photofact TR-49. There's also a factory set, but I haven't purchased it yet. The owner's instructions can be found on my manuals page.