What is, and how to get, the name of kit/part of the tripod that missed out ie. came off from it as shown below in order to have it ready to be mounted with a common/cheap Canon auto focus camera or preferably all/most common cameras of that such and the manual focus too.
It's Mars PT-21 (likely discontinued) tripod product

Hopefully one can correctly guide me to the market name of this part common people recognize or are familiar with, preferably would elaborate its info detail.
Thanks much before