My lens has the same markings and tripod foot as OPs, it says Danubia on the front lens: DANUBIA 1:8 f = 500mm ø67 LENS MADE IN JAPAN and the serial number
Some more info on these kinds of lenses here on pentaxforums:
Pre-set (Quantaray, Kimunor, Five Star, etc.) 500mm F8 Lens Reviews - Miscellaneous Lenses - Pentax Lens Review Database
I got one of those myself for 15€ shipped, it is not looking good, one of the previous owners tried to get to a dusty element himself it seems and glued together the rest later... it's wobbly as well.
I have only had one walk with it so far, at f/8 it's quite soft, a little better at f/11. I used it handheld on my K-S2, so I had to use a really high shutter speed and really high ISO. In fact the ISO was at 6400 with the shutter speed at 1/320 s and the image was still underexposed. Overcast day. This might have been the best image I got with it, pushed the shadows quite a lot in DxO. CA correction sliders all the way on the right. Little detail left on the dark bird. Just reprocessed it with DxO 4 with DeepPrime noise reduction, looking a little better than the previous blotchy result with "normal" Prime denoise.
Eurasian coot
For comparison, this is an ooc jpeg from the same day, can't remember if f/8 or f/11. Judging by sharpness I would say f/11, judging by the longitudinal CA I'd say f/8. Focussing is difficult, I'd say almost impossible to get right through the viewfinder, and the sensor stabilisation goes crazy in Liveview. But you can easily judge the focus setting by looking at the CA on a picture like this:
Egyptian geese
I've got no experience using focal lengths like this, and it was an overcast day, so with a tripod or monopod and good light I think you can get some good pictures out of it, but I'd say it's not an easy lens to use (all manual) with good results being difficult to achieve, but it's very lightweight and doesn't cost much, so it wouldn't hurt to try it out.
Apart from the reviews on this site, maybe you can find something in German when looking for "Wundertüte" (grab bag) and "500"
Last edited by ehrwien; 03-10-2021 at 12:07 PM.