Welcome to TBN:thumbsup:
I can't directly answer your question, but the TO20 was similar to the 8n fords. And they kinda had a parking brake. Their was a pawl with teeth that enguaged and held the brakes on. It was all external and located under the seat on the side of the transmission/rear end housing. It was supposed to engage by stomping the brakes quickly, but that didn't always work and sometimes you had to reach down there and manually engage it.
But again, I am not sure if the TO20 is similar or not. Just follow the linkages and see if anything looks like it will lock or hold.