66B125
Was it larger that your standard 66 block, and did not have the standoff bracket? Typically used in 1A2 installations for the "feature" block, where you brought a cable from the KSU to the side with the pins facing differently from all the others, and then ran jumpers from the other side to the blocks that held the cables leading off to the phone sets. In 1A2 you ran a 3-pair jumper from the feature block to the station block for each line that was to appear on a button on the set, then other jumpers for intercom, busy fields, etc.
Now days, it's good if you need to jumper the same line or lines to several cables, again because you can have the line land on the block once on one side, and take up to 5 jumpers to other cables from the other side.