Had I known the correct way to ask the question, I could have found the thread size by Googling it.
If you know that the fitting needed is called an "inverted flare", and you know that you need a 3/8" inverted flare, you can find the correct thread size from this chart, which I just found on Google.
http://fittingsandadapters.com/threadchart.html
When I took my pump to town, I didn't know that the hose barb I needed was called an inverted flair, nor that is was 45°. I didn't even know it was called a "barb". I was looking for, & asked for, a hose "nipple", for 3/8" hose.