Panel Sender with doors open exposing relays inside. As can be seen various types of relays were used in Panel Senders.
<a href="//commons.wikimedia.org/w/index.php?title=User:Panel_Switchman&amp;action=edit&amp;redlink=1" class="new" title="User:Panel Switchman (page does not exist)">Panel Switchman</a>
Early 1970's
<span class="int-own-work" lang="en">Own work</span>