Sorry Wouter gotta disagree here. In the UK, (IMO) people will always use the multiplier first.

Back to this multi purchase...

in the picture I agree that up to the last block on the boom, there is a an 8:1 system.

But I do not understand where "only" an extra 1 comes from.

The block is moving down, and there is a (at least a 8:1) moving over this block, BUT as this block is moving itself there MUST (I think) be some multiplyer on the rest of the system. I agree it cannot be 2 as there is not a 180 degree turn, but I cannot see how it can be 1 as the block on the boom is moving.