My old (gee, it was brand new when I bought it in the 1960s! Why is it old now?) Cub Scout knife has a rounded brass lock tab, but it's for the awl or the screwdriver/bottle opener (which was so stiff I couldn't get it open as a child), not the spearpoint cutting blade. Never made sense to me that the blade didn't have a lock, but the tools did. Mine is by Ulster or Camillus, I think (can't get to it to look at the moment).

BTW, Buck has been in business for over 100 years, so they probably were around back then... :-)

David