When was the thrie beam introduced?

[Tom958]

A pretty straightforward question. What prompted me to ask was that, in the course of preparing this post about the early Interstate bridges of Idaho, I discovered that thrie beam bridge guardrails are extremely common there. Many states augment or replace earlier, unsatisfactory bridge guardrails with thrie beams, but I found this apparently original thrie beam installation on an otherwise-normal 1971 bridge. Other nearby bridges built in 1971 sport normal guardrails. Now I'm wondering how many of the zillion thrie beam bridge guardrails in the state are also original equipment and even older.