#1 cam follower block fitted but not yet bolted down. It is very important to keep the pins in place, to hold up the tappets, until everything is tightened up. The blocks require gently tapping down onto the locating lugs. It should be noted that where the cam lobes are in position to fully open a valve the tappet will be raised off its pin by roughly 1/16". If the block will not seat, due to this positioning, the shaft should be rotated slightly - do not try forcing things.
It is possible to see when the blocks seat against the bottom of the chest (just!). If there is a gap that will not go away check that the bushes ('B' in the picture) are correctly aligned.