The cable extension on the top of the boom, is similar to the old skyhook/ skyhoist and wilkie sign trucks. But neither of them ever used a top mounted extension cylinder. And most of their stuff is round boom.
But its built along those lines, think less boom truck and more sign truck. But there's been a lot of small builders come and go in the sign truck industry.
The controls on the side of the turret is a typical sign truck thing. Most of the boom trucks are going to have a platform.
If it doesn't have a manufacturer name and load chart, you'll never get it to pass any kind of inspection, that a insurance carrier would want, to lift items for other people.
Which is likely why its for sale, i.e. not that there's anything wrong with it, its just no one can write down a manufacturer name on the inspection paperwork, because no one knows who built it. No inspection= no insurance = no crane.
It actually looks like a pretty well built unit. But I think you would have better luck with searching "sign trucks" and google images, and sign trucks for sale, if you want to find a manufacturer. Or even the right old guy in a old sign shop might look at it and recognize the builder.