The American Sellers coarse thread is a copy? of (the same as) the
Whitworth coarse thread.
The thread angle is different - whit. 55degrees, Americian 60, but
all are
interchangable as all pitches line up accept half inch where one has
12TPI the other 13.

Before our country went metric this caused considerable confusion
amoung engineers not in the know and still does for our vintage
restorers.

Doug
New Zealand