Old valves had 2 holes, or sometimes a slot in the top. The tool fits in the 2 holes. You put grinding/lapping compound on the lip of the valve and spin the valve around(back and forth) with the tool to finish fit the valve to the engine head.
Modern valves and more accurately machined, and self rotate in use, so they self lap after installation for the most part. Valve lapping tools are more or less obsolete except for small engine repair & antique restoration...
Your tool would probably need a bar rather than a ratchet, so it can be turned both ways. The screws are for setting it to match the hole spacing in the particular type of valve you are lapping...
The tool may be a clue on the sockets, as there were a relatively small number of companies making those things compared to the number of folks making socket sets...
There is not all that much functional difference between the valve lapping tool and a face pin spanner, but the 2 piece stamped metal design with a screw is very common for valve lapping attachments..