Blogs,
I think you better do your homework re. the 250kt rule.
I don't think the DC-9 even existed when the first 250kt rule came in about early 1960s.
However, it could have had something to do with a collision between a Connie and a B707 east of New York.
I was flying around there in that period, by 1967, it was there then.
Tootle pip!!
Correction, it was a DC-8.