The lower portion of the front panel and the removable panel on the back were used for the installation of optional camera controls or a 4-channel switcher intended for field applications. The MI- number on the back panel of these units will vary depending on configuration as shipped.
An article introducing this unit appeared on page 69 of the December 1959 issue of RCA "Broadcast News". Available here:
http://www.americanradiohistory.com/...ws/RCA-106.pdf