Yes trapezoidal would be counter part name in metric for acme since Imperial uses names instead of description
I admit I missed the word trapezoid in rick's original post the 13 TPI was what hit me in the head. that and the 16 mm dia x 2 mm pitch
I am now thinking what he found might have been a nut for a worm screw in which case the worm would have had a pitch to match the corresponding gear
since it would be impossible to use a 1 or 3 wire method to determine the exact internal thread of a nut I guess the use of a ball would have worked, however the point of your post is you were able to help a friend out without having to expend more energy and time than the task was worth.