I'm embarrassed to say I was a toolmaker for a long time before I heard about external hones. There came a time when I found I needed just such a tool so I made one. It uses a replacement stone from an automotive brake cylinder hone but others could be employed.
There are times when you need a long straight bore in the lathe and a between centers boring bar is the right tool.
I ran across a design that implemented a shop made cartridge that could be advanced with a graduated screw for size setting.
The clever design (not mine) captures the screw in a smooth bore, but turning the screw advanced the cutter via the partial thread in the toolholder cartridge.
Everede makes a line of conventional boring bars utilizing a triangular tool bit. Clearance angles are automatically taken care of. This was a good application for the small space. The triangular space was produced by brazing an assembly and then tapping and turning down to size.
This could be either a indicator for carriage movement or an adjustable hard stop. This is an Atlas lathe with flat ways so the clamping mechanism works front to back.
The stop can be mounted on either side of the carriage as needed and the indicator swapped for a shop made micrometer stop.
For a smooth clamping mechanism without a tendency to shift the position, this has a simple eccentric with a small bearing mounted.