I think I have found out what this tool is used for. I understand that it is used for setting up the cutting tool at the correct diameter for reboring an engine cylinder using a BUMA boring machine, frequently ‘in situ’. It is probably the standard tool that is purchased with the BUMA 'APM' model. I attach a photograph of one type of BUMA boring machine. The cutting head can be seen at the base of the machine.
The design of the tool allows the cutting tool holder to sit on the round metal spigot on the top face. Then to make sure that the cutting tip is aligned with the centre of the micrometer arm, the sliding metal bracket sits in front of the cutting tip. The bracket is designed to slide so that it can be used with different length cutter holders. From my researches there are such machines still in use along with American Van Norman machines. I also attach a photograph of a BUMA cutting head with the micrometer tool being used to measure the cutting tool diameter.
My plan now is to list it on E-Bay in an attempt to find a BUMA boring machine owner who needs a setting up tool!