For me it's a small two blade jack of some sort.
When I was a kid, I had a foot in both worlds. My dad moved us to Washington D.C. for his job just after the war. But when I wasn't in school, I spent a lot of summers on my grandparents place down on Maryland's eastern shore. Our place in Washington was city, but the eastern shore was very rural coastal waterman area. When I was 'down home' as I thought of the shore, it was a pretty knify place. There were hunters, (really poachers, but they didn't think of it that way,) working watermen, and farmers. They all carried a knife, and if they were not hunting, the knife was usually a small two blade jack of some sort. Barlow, serpentine jack, equal end jack, or even a trapper. there was muskrat skinning beong done, and the trapper was the perfect knife for the job. If they were hunting, a small leather handle Little Finn type sheath knife was worn, but for the most part, a two blade jack was the universal knife of the day.
In the city, Washington D.C., and later when dad moved us out to the Maryland suburbs, the two blade jack was still the knife of choice for most men of that era. Delivery drivers, postmen, store clerks, newsboys, and tradesmen, all carried some form of two blade jack. Sometimes a war issue TL-29 that had come home in a pocket was used. When I think of the knife of my childhood, the two blade jack was the ubiquitous knife of the post war times. An office worker may carry a small jack like a peanut or dogleg, or even a giveaway jack from a hardware company or car company with the logo on cracked ice celluloid scales. My own father carried a Case peanut, and was loyal to the type, using his little knife for any cutting job that came up.
Until the 1960's and Buck knife, the two blade jack was king.