The valve springs are installed only one way.... I'm not sure what you are looking at, but if you look at the spring from the side you will see one end has a tighter winding of the spring then the other. The tighter winding goes down.
You're right. Don't know how we could overlook this! Even Toyota in Holland didn't know it can be upside down! I ended up with removing all valves again and check for the correct mounting. I'm getting handy at it

Only thing i wonder about is about the valve spring retainer locks. Sometimes they just don't line up completely straight. So at one side they are side by side while on the opposite side there's about 1 mm distance between them. Some line up perfectly of automaticly line up perfect after valve turning. Sometimes they were lined up perfectly but after a bit turning of the valve they aren;t anymore. Let's do the next picture to the talking:

I was talking about the fact that the upper side of the spring has a sort of an arrow on top. Down it is more flat. Let's show some more pictures to clarify what i'm saying:
Upside (difficult to see but if you take a good look you see an arrow poining to the right):

Down (this one doesn't have an arrow pointing):

@ this picture you can see the difference in windings. Unbelievable we didn;t spot it before

:
