Here are the photos of real screws:
http://img695.imageshack.us/i/toolslideleadscrew.jpg/
http://img687.imageshack.us/i/horizontalleadscrew.jpg/
Definitely they are square screws.

I measured again:

tool slide screw:
TPI = 6 (I know, it's unusual, but it's as you
can see in this picture http://img72.imageshack.us/i/toolleadscrewmeasure.jpg/
)
D= 20,1 mm ( i have make this measure again and this is
the value)

x horizontal screw
TPI = 4
D= 25 mm (I'm sure that it's not 25,4mm , perhaps some
clerance...)

Definitely TPI is 4. I made a 4" lenght on the caliper stopped and it
fits very well with 16 threads, so 4 threads per inch. Otherwise i
have made a lenght of 100 mm on caliper and it's clear that it's very
different form 16 threads.