Yam TT500s, modified for flat track, a popular entry-level 'vintage' bike in the US. There are tons of them, with modified, deraked stock frames. They're nowhere near as popular in the UK and Europe, because TT500s are relatively expensive here. They are effective though. A 500 Yam isn't a Vintage class bike in the DTRA, but can compete in Thunderbikes.
This one looks to us like a recent import that someone is trying flip for a quick profit (not that there's anything wrong with that). The spec drops lots of bizarre references, like 'DTRA-spec bodywork'. It also says the forks are Cerianis. I don't know for sure, but they looks like shaved common or garden road bike forks to me. That looks like an 18in rear too, not 19, so fitting easily available race tyres wouldn't be possible.
Somehow this imported race bike (with no front brake) has passed an SVA test and got a licence plate and logbook.
This is an imported race bike, so buyer beware. Expect to do at least a top-end rebuild and then, if you don't have to you've got out of it with credit. Thanks to Kev H for the tip-off.
It's up for £2995 and is located in West Yorkshire.
To buy or investigate, click 1977 Yamaha XT-500 FLAT TRACK / BOBBER / TWIN SHOCK THUMPER, AWESOME MACHINE!! (the seller's words, not ours)