Hi!
Your TUEC 260 was one of United Electric's later designs, and made use of a universal-type motor instead of the split-phase motor that the earlier machines (at least the single phase AC ones) used. It's altogether quite a bit smaller than the earlier TUEC units, as well as units of competing brands. It was designed to be a very cost-effective way to incorporate a built-in vacuum cleaner into a home, owing to its simple design (one fan, no filters). This simple design, however, also made it longer-lasting than other makes, and the comparatively larger piping and hose made it more able to inhale larger debris without clogging. However, a single fan coupled to a 5000 RPM motor, even though it moves a fair bit of air, does not generate very much waterlift (think 20-30"). It'll dust your bare floors for you, though.
The inlet cover and key for the front of the machine is not something I have been able to locate (besides the ones that are attached to my machines -- and you will forgive me for not wanting to part with those!!). But, as MadMan said, a big rubber cork about 1.5" to 1.75" OD would do the trick just fine. The inlet isn't threaded, so the original plug would have been just friction fit. Your wall inlets, fortunately, are of TUEC's fancier hinged type, so you can't lose the plugs for those.
It shouldn't be hard to get a new hose that will fit your wall inlets, just need to do a little experimenting with the cuff size. 1 1/2" ID or at a minimum 1 3/8" ID is strongly recommended, and as short as possible to reach everywhere from your wall inlets. 25 feet would be great. If you try to put 50 feet on this system, there will be veeeeery little air flowing through the end.
I have attached an advertisement for your 260, as well as a picture of my 260 (the seller's picture before I went to pick it up -- disregard the hose and attachment on it; they are from a 1940s Hoover). I made a Youtube video about it too, which might interest you. It would be cool to see yours run!! They are nice and quiet if the exhaust is connected.
One last thought -- on two of these old systems that were vented into the chimney, enough debris fell from inside the chimney into the upturned elbow of the vacuum's exhaust pipe over the years to clog it. So if your machine seems to be moving very little air, that's probably worth checking.
Hope this helps!
Owen
