Gerhard,
You might find both BSF and BSW in use on the same machine, or even as suggested, an earlier version of SAE threads. It most probably depended how good the design engineer was, and the availability of ready machined fasteners.
The only definite way is to get armed with a set of thread gauges and a micrometer, a sheet of threading charts or a ZEUS book, and sit down and measure them all.
If the bolt is going into a steel part, if might be BSF, as the steel has a higher tensile thread strength than cast iron. So it might be BSW when threaded into cast iron or non ferrous materials.
When I was restoring an old Myford, I found it was easier to make my own bolts out of hex bar rather than chasing about getting bolts with the correct head form, thread and length.
That has most probably confused you even more.
Bogs