Here's a start. I still need to locate a few, including one I had in my pocket just yesterday.
Hawkbills:
Tang stamps: "M. KLEIN AND SONS CHICAGO MADE IN USA"
Model: 1550-4
Maker: ? (guessing Utica)
Date codes: none noted. Thus, likely made before 1972.
Handles: smooth Delrin (I'm unsure if they were originally woodgrain at one point)
Notes: one of the few that I own that either I didn't buy new, or knew the previous owners--came in used lot 5-6 years back.
Tang stamps "M. KLEIN AND SONS CHICAGO MADE IN USA"
Model: 1550-4
Maker: Utica
Date codes: J (1973). The man who gave this knife to me would've barely been a teenager when it was made, but he says the mines supplied it to him. Guessing the supply house had old stock, as he started working in the early '80s.
Handles: woodgrain Delrin
Note: original owner was a friend of mine, who used it in the mines. Like three others, it "had been a toolbox knife for many years that I found in one that I was cleaning out"
Tang stamps: "KLEIN TOOLS CHICAGO USA"
Model: 1550-4
Maker: Utica
Date Codes: V (1984)
Handle: Woodgrain Delrin
Note: was my Granddad's issued knife in the mines, and later a yard work knife. Eventually became my work knife before being retired in favor of a newer one from just a few years ago. Supposedly, this one was found, and had been ran over with a piece of mining equipment. Yup, these are tough.
Tang stamps: "KLEIN TOOLS CHICAGO 1550-24 MADE IN SHEFFIELD ENGLAND"
Model: 1550-24
Maker: Ibberson, based on their Endurance series.
Date Codes: None.
Handle: blue plastic, feels like polyethylene (PET).
Note: Bought new by me in November, 2019. I would not recommend these. I pruned one thin branch and the pin loosened. These are an integral frame/hiden backspring design held together by one measly brass pin and no liners.
Tang stamps: "KLEIN TOOLS INC JAPAN" (mark side) "44004" (pile side)
Model: 44004
Maker: so far, my evidence points to Moki, based on their Gemini line from the '90s
Date codes: none (seems like if it ain't made in the US, they don't date them)
Note: bought new by me in 2021, and has been used regularly since. This was one of two that replaced Granddad's--little Japanese-made one for EDC, big US-made one for real work. These 44000 series knives are well-made, well-priced, slim, light, and tough. They take everything from "laying around the house in sweatpants" to "whacking brush you can't get at with the weedeater" despite being small. Most of the time, I have the thumb stud swapped over for left-hand carry. I'm thinking about getting one of the bigger ones with a non-reversible clip that will have to have the stud left for right-hand use as the clip is right-hand only and non-reversible.