With no model numbers, we have narrowed the time frame to 1952ish, when the stamp went to UC/LC Italics, and 1966, the end of the no model number era.
The earliest date that a knife can have Delrin is 1961, when Dupont first made it in commercial quantities. So time to do a celluloid test. One such test is to take one of the knives (closed of course) and briskly rub it between your hands to warm it up. If you can smell camphor coming off it, it is celluloid. Wash your hands between knives to prevent a false signal. Presence or absence of camphor will indicate which time period each is from.