I wonder if they are the same blocks, that Boeing keeps in stock.
They are probably not the same concrete blocks that they used for the JT9D, but Boeing does have a collection of concrete block 'engines' that they use - different weights for different engine types.
It's not always an engine problem that results in their use - often times aircraft are 'built ahead' - rolled out well months ahead of the planned delivery date - to even out the production rate. They hang the concrete engines then push them out to a remote part of Paine Field until the delivery date approaches then they are brought back and the engines are hung (wide body engines are in the $12-$15 million dollar each range, so there would be a large inventory cost associated with hanging engines that won't deliver for several months).