I answered a Craigslist ad for some broaches and found that the seller is a retiring machinist emptying his shop. So poked around for some interesting bits and found 3 lathe holders similar to this:
Kennametal holders in C size, where the PO had machined the shank down to fit B holders. You can see where I took a bit more off to fit my A holder. The round insert is called a K-lock, and is held in place with a 50-degree taper, plus normal stock rotation is downward force. To change you hammer it out with a pin punch from the bottom. Kennametal catalog says these are good for large DOC. I didn't try that out, but they do give a nice finish on steel.
I can also do both turning and facing without moving the holder!
One of the other holders uses a 1/2" diameter insert. Either can be used as a form tool where appropriate. I saw Stan Shire doing so in another thread.