I just bought a G0704, and an Enco lock-down vise. The vise appears to have the type of thread you mention. It looked funny to me when I first looked at it, but assumed it was just my bifocals. I can't take a good look right now, as I don't want to tear down my current tooling setup.
I did look at the X traverse screw on the table, and the Z screw for raising the head and they both seem to be Acme-ish threads. Flat tops and bottoms of nearly equivalent width, and a much shallower angle than 60 degrees on the flanks of the thread. The table thread has to be 10 threads per inch, as one turn of the dial gives .1" of table movement(no DRO yet, still counting turns).
In general, the mill seems pretty satisfactory. The dovetails are nothing to write home about, but the machine seems to function well. The headstock seems to be well aligned in the Y direction relative to the column, and it was easy to get the X direction parallel to the column. The head came from the factory with the 3-bolt modification. Tramming the column in the X direction went well, but was surprisingly sensitive to the bolt torque. I'm making little rectangular parts and I can hold .001 end to end on a 1 1/2" part when milling in the X direction.
The Y direction has problems. The table holds .0005 under the quill using a dial test indicator as it is moved in the Y direction, but tramming the column shows .0045 across the table. E-mailed Grizzly, and the spec is .005 or better...... Have to finish the little job I'm working on where this inaccuracy isn't important and will work on shimming the column when my shim pack arrives from Enco.