I need a source for 1/2-8 LH Precision Acme Lead Screw for the cross
slide of my Sheldon Lathe. The specs are listed below for the screw. I
have a .125" direct reading dial. I think it is a single start screw
but am not sure.

MSC has double start 1/2-8 LH, but what is the difference in single and
double start?

Here are the specs on the screw from someone I know who worked for
Sheldon:

The thread is an American Standard 29 degree Acme GP thread, 1/2"-8
left-hand. The screws made by Sheldon had a 5 degree 25' helix &
measured
..550" to .552" over 3.072" wires. They were made of Stress-proof 1/2"
ground & polished stock.

Any suggestions greatly appreciated.

Thanks, Steve