from the transmissioncenter.net web page:

350 / 400 Vacuum Modulator. Adjustable, allowing you to tailor your light throttle shift points with-in a range of 2-4 mph. Supplied adjusted for normal shift points. If your engine doesn't make adequate vacuum you will not be able to make the stock modulator work properly, usually resulting in late, hard shifts. Cost $15.00.

I thought that not all modulators had the adjustment screw. IF you remove the vacuum line, the shift is delayed (or not at all)...

"Of course, I could be wrong, I thought that I was wrong once but I was wrong."