Wow this is all great advice. I've examined these blocks pretty carefully and can't find any adjustment, but I'll definitely take another look. I've leaned on them very hard and not gotten any locking in either direction but they do seem to click in both directions. They are six 'sided' harken pulleys with a small red plastic insert on each side. And they do have a directional arrow. I am assuming they are meant to ratchet. I'll do my best to give them a good cleaning.

Thanks everyone for all the help, Duncan