Looks like this one?
I paid $2500 in 2007. It didn't need anything then and looked to be all original. More recently I have replaced the seals in the outer boom, and it could use one hose that is getting chewed up at the base of the boom.
These were made by a US company and sold as OEM by both Kubota and Yanmar-USA as original equipment, matched to 20 hp tractors. Years would be late 70's into early 80's. I think the K to L model change would be about 1980. So far as I can see the only change was a new manual with all new Kubota part numbers.
These came with a pto hydraulic pump. You can run it from the tractor hydraulics but I think you need over 5 gpm or its going to be awfully slow.
As for productivity - These are as big as you can go before overstressing a cat 1 3-point, but the short arm and limited crowd (pull-in) pressure are not as productive as a frame mount backhoe with its better specs.
Mine matches what I need for orchard maintenance - to remove stumps and to dig out weed oak trees that have started so close to the apple trunks that discing can't hit them. Note the size of the root (with glove for size reference) in the foreground. The backhoe pops these right out without hurting the apple tree roots. If you need to dig long trenches you want a frame-mount with a longer boom so you don't have to climb down and move the tractor every 4 feet.
As for your pool - this would work fine for roughing-in a pool excavation but you don't have enough height or reach to swing and get rid of that large quantity of excavated soil. You would need to use your front loader to go in the excavation, scoop out the loosened soil, and carry it away.
