https://en.wikipedia.org/wiki/Main_distribution_frame

The backboard that is centrally located, equipped with terminals where all your key system, station, and central office cables and connections appear. It's where you make the cross-connections that bring dial tone to the equipment, and features to the stations.

https://sundance-communications.com/forum/ubbthreads.php/topics/568832/Photo%27s_of_large_1A2_system_an#Post568832

Here is a posting from this very 1A2 sub-forum from earlier today. It is from my friend Phil who has a huge PBX and key system collection. Click the link that he provided and you will see a large key system MDF.

Arthur P. Bloom
"30 years of faithful service...15 years on hold"