I had a old post hole shovel that went in straight like a regular shovel then it had a lever on the top of the handle you pulled to change the angle 90 degrees to clean out the bottom of the hole. Most interesting shovel I ever owned. Sure wish I had not sold it in the downsizeing move. I live on a big lava rock now and a jack hammer is the hole digger, then you fill with dirt, then you plant.
I was introduced to these when I worked (while young, stupid, strong, energetic) for McGee Fencing in Ottawa in 1974. I spent an entire summer digging holes in a 40 mile circle around Uplands International Airport for installing 8 foot chainlink fence. These shovels (called "clam digger" by the fence installers) are absolutely unbeatable and I even had one extended 30 years ago so it would go down 5 feet. They make a perfect 8 inch diameter hole with a clean bottom. Meaford, or maybe is was Erie, of southern Ontario manufactured these and both of these companies are now defunct and have been bought out. The current hard-to-find 'clam digger' looks much the same as the 30-40 year old versions but they don't dig for very long before they break. Streamlined design with many more pressed steel parts and no more riveted joints. I will try to take some photos.