Delrin was in use for probably 5 years or so. It was around longer than most think. I have had seven spacer knives and three thick two think spacer knives in Delrin.

I would say probably from around 66/67-71/72 or so.

I once had a "SS" stamped model 7 in Delrin, but attribute that to a "deep bin" knife if you will.

You note the change to Westinghouse micarta, as the very earliest examples have 7 spacers, with very few examples out there, but you see only limited use of the Westinghouse material in the three thick two thin period, 73-76/77. The micarta shows up in more quantity in the thick center spacer period say from 76-77 onward. It is only my thought, that Bo knowing the Delrin was unsatisfactory, was always looking for a better mouse trap, and got some samples of the Westinghouse material in to try. Exhausted that limited supply and eventually settled on it.