Doing valve work on a flathead motor was always a lot of fun: on many, maybe most flathead motors, the access panels were hidden behind the exhaust manifold, and weren't very large. So you were reaching into an oily, hard-to-reach space, unable to see what you were doing at least half the time. These valve spring compressors/valve lifters pressed against the cam followers or the block on one side and the valve spring retainer on the other side. Closing the "hand grip" side opened the tool on the other, with a little pawl-and-catch arrangement holding them open*, so that you could remove the valve locks (little chunks of steel that fit between the retainer and the spring and are captured by a neck on the valve stem and held in place by spring pressure). Reassembling the valve reversed this, with all the entertainment value of placing little chunks of steel in place by touch (remember: you can't see anything) and hoping they don't fall out while you release the valve lifter.
Back when gasoline was low-quality and the motors low-tech and you regularly pulled the head to decarbonize the motor and lap in the valves, you'd get plenty of practice in doing this; the reason that valve lifters show up regularly at yard/estate/etc. sales is that they were commonly found in the garages of homeowners who did this work themselves, every few months in some cases.
Valve adjustments were just about as much fun: narrow space, can't see what you're doing, the adjuster on the cam follower and its locking nut thin enough that you needed tappet wrenches, with thin heads and long bodies (so you could hold them in that confined space).
Overhead valves yield a more efficient engine, but they were also a godsend for the mechanic: you could see what you were doing.
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*Unless you'd gone first cabin and purchased the type of valve lifter that was operated by a screw with a handwheel-type nut.
