I picked this up at a flea market.  It says Millenium Parallel Valve lifter.   The mechanism goes up when you screw the butterfly nut in.  I am trying to understand exactly how it works.

This would be for use on a side valve engine.  Valve access is on the side of the cylinder head.  The valve head faces up, the spring and keepers are facing down.  The tool goes in flat side against the bottom of the valve chamber.  The slotted side goes under the valve cap and lifts it off the valve end, freeing the keepers. 
I used this type lifter on old Ford and Massey Ferguson tractors.

side valve = flathead, 
English to American translation <smile>

Side valve = flathead =L head engines,  can think of 7 different L head engines, pre 1966.   Old valve spring compressors have been finding me lately. 

old goaly, Ha Ha!  It figures - I have 4 Brit bikes in my garage (though none are "side valve).

I have a few old ones, a couple of more with Villiers 2 stroke motors.But My shoulders are bad so I stay off of them.

Quote from: john k on October 24, 2022, 11:55:39 AM...can think of 7 different L head engines, pre 1966...

And, of course, there were way, way more L-head/flathead/side-valve engines than that over the history of internal combustion engines.

Rambler offered them up to 1965, says one source on the interweb; I believe it was the last American manufacturer to do so, unless Checker, with its flathead Continental engine was later.

Harley continued offering a flathead motor on its Servicar (three-wheeled motorcycle, used by many a parking enforcement officer) until 1973, because motorcycles avoided emissions controls way longer than automobiles.