6.85×40NA-55？ I came across a picture, and the thread mark was 6.85×40NA-55. What is the specific explanation and what is the standard?

“6.85×40NA-55”，

I think maybe 6.85 is wrong, it should be 6.35, which is 1/4 inch;

40 should be the number of teeth per inch; However, this is a bit contradictory. The number of teeth is 40, which is generally fine teeth, but the thread diameter should be 6# (equivalent to metric M4) or 4# coarse teeth with 40 teeth.

NA should be accuracy grade n,1a.

Another possibility is that the marked thread is nonstandard.

6.85 Nominal diameter, 40 teeth per inch.