It's an English Ford 8hp / 10hp sidevale valve guide remover. The only way to remove the valve is to remove the guide as well.
The valve has a ball shape on the tip that accepts a 'c' shape spring retainer, thus the valve will not pass through the guide. The guides are a two piece split item. You position the tool over the valve head ( hence the hollow area ) and tap downwards to release the guide. ( similar arrangement on several old sidevalve engine types )
You then (having previously checked the valve clearance) adjust the clearance by grinding the valve further into the block if the gap's too big or if the gaps too small, grind a bit off of the valve tip. Those were the days !
A picture is probably easier than my dodgy description.
Regards Steve