Got this thinking the rods could be connected (they cannot) and it was a very long inside micrometer.
I cannot for the life of me find anything on the internet about how these were used.
It was described as a Lufkin (the make) inside micrometer set, but I’ve also seen it described as end measuring rods.
I think (although I’m not certain) that it’s an inside mic set with some standard gauge blocks, so you could find the distance along e.g. an internal corner by laying the gauges along it and using the mic there . The alternative would be if you use it for setting up a standard assembly where you’d need a standard gap of 1"/2" etc.
After a bit more sleuthing around I’ve also read that these we’re used on lathes, stacked in a tray lengthwise.
https://www.practicalmachinist.com/forum/threads/jig-bore-gauge-rods.322399/
And some pics of how they’re used.
http://www.lathes.co.uk/matheys/
So mystery solved.
