Momus is correct re the travel of the tailstock per revolution. Although both 2.5mm pitch and 10 TPI are very similar. 2.5mm pitch = 2.5mm lead (distance per rev), 10 TPI = 0.100" lead (2.54mm per rev).
If you can't work it out by the graduations on the handwheel, put a dial indicator on the tailstock quill and measure travel per one rotation of the hand wheel.
Other differences between trapezoid and ACME threads is basically the included angles, 30 degrees for trapizoid and 29 degrees for ACME and some thread depth variations. ACME is considered an imperial thread while trapezoid is the "metric" equivalent.
TTezza is also on the right path with where the lathe is made, if it's an Asian made machine it's more likely built to metric specs so it would probably be a trapezoid thread. If it's an old UK or European ACME is likely. Measure the OD of the tail stock spindle thread (where it's not worn), assume it will be slightly smaller than nominal for clearance purposes and go from there. If it's 14mm nominal it would probably measure about 13.95mm, if it 9/16"(14.28mm) nominal it will likely be about 14.20mm.
Regardless of the above, making a nut of this size with an ACME or Trapizoid thread is difficult at best. I sell machine shop (lathe and mill) tooling for a living and when I have someone ask me for tooling for these types of threads I ask these questions:
What thread form?
What thread pitch?
Thread root diameter?
Length of thread?
R or l/h thread?
material (steel, SS, cast iron or brass)?
Given it's left hand (nearly always special tooling required), has a root diameter of around 9-10mm (too small for most standard tooling with that pitch), most likely to have a thread length of about 20mm min (exceeds length /diameter ratio), I would recommend tapping as the easiest option for this type of thread given it's likely to be out of brass or bronze. It is possible to grind a small HSS tool and screw-cut it but unless you know what you're doing, you may not be successful.
You could get a special tap made by Capital Cutting Tools in Victoria Phone:(03) 5571 1772 or make you own if you're able to. To do this use a piece of 4140 round (high alloy steel) or similar and machine a L/H 14mm x 2.5 pitch OD thread, turn a taper for the lead-in and mill a few flutes along the length that give rake angles like a tap suitable for cutting brass / bronze. Because brass is easy to cut and it's a 1 off thread you may not even need to harden the tap, but if you do need to you can heat with an oxy to dull red and quench.
Alternatively if you can get hold of an old tail stock spindle, you could make a tap from it by doing what I described above.
The other obvious is, are OEM spare parts available?
Keep us informed on what you end up doing.
