Almost universally it was some sort of small jack or pen knife about 2 3/4 to 3 1/4 closed. This was the 1950's, and everyone carried some sort of small pocket knife to cut twine or open mail or cut whatever. They were so universal that every five and dime store had a small stand up cardboard display up by the cash register with the little jack knives on them in rows. Usually had what was called cracked ice celluloid handle scales or plastic fake pearl handles. The stand up displays were right alongside plastic combs and nail clippers.
What was weird compared to now was, when I was growing up, if a man had pants on, he had a knife in one of those pockets. It was just a given, like the sun rising in the east, it got cold in winter, and a man had a pocket knife.
Once in a while you saw another kind of knife, like the Italian style switchblades the would be punks carried. All the gown men I was around then, dad, uncles, scout master, had great distain for what they called a "punks weapon." If they needed something more than their little pen knife, like going hunting, they carried a 'huntin knife." Likewise for fishing, they carried a fishing knife. The men who came home for the war, WW2, just carried their old issue Camillus or Kaybars to go huntin or camping. But for everyday they just carried the little two blade pocket knife the size go a SAK classic on the keychain. If things got rough and dirty, the old Stanley 99 got used. But the little pocket knife got used for a heck of a lot. Most times it was all they needed.