Mr Hyde, the 612 box was made for a loooong time. It began life in 1937 as the bass enclosure for the Lansing Iconic. It found a new role in 1945 as the utility enclosure for the 604 Duplex, and thousands of this combination were used as studio monitors.

I have four original Iconic boxes, and their dimensions all vary slightly, like 1/8" here or there. I measured one and drew up plans for someone several years ago- here they are.