I think they would be good on rigid insulation, and many other things. The blades are very thin and flex. The reason I think they are one offs is because they are so poorly made. For example the blades are very unevenly sharpened and the screws holding the handles on are uneven and many also have cracked the handle and some handles are somewhat loose.
Now, as to what they were likely used for by the owner. The owner was in the business of servicing cell towers as well as building them. The used ones are covered in some kind of heavy silver/cadmium paint. I'm guessing these were used by him for scraping off old paint. Some of the old ones were dull and others had been resharpened many times.
I have given some of these to a number of friends and they are finding many uses for them.