If you notice in the diagram there are 6 dowel pins between the cylinder head and the valve cover. 
(shaded in red)

These dowel pins are free floating, meaning they are not
anchored to anyone cover, spacer or head.

As you remove the Valve Cover, spacer plate, or rocker
arm base the possibility of one or more of these dowel pins
will drop down and fall in to the valve area or worst
into the engine oil return port opposite side of the putrid
area.

When removing for the first time anyone item, look care-
fully as you lift upward to see if a dowel pin might be hanging downward. If so, carefully slide in some paper
or use your fingers as to not allow them to drop down and
into your engine.

To insure they will stay put next time you venture into the 
engine use a drop of blue loctite upon each dowel. 
2 ea. in the head, 2 ea. into rocker arm base, 2 ea. into
the spacer plate.

Next time the dowels will remain in the lower piece of 
hardware and limit the possibility of losing one of them.

If the need arises to remove the pins a little heat or a pair
of needle nose vise grips will remove them easy enough.

The Dowel Pins are appox 50 cents each, Cheap enough
if damaged from removal and cheaper than fishing for one
in the bottom of your engine.