Because eg BSW and UNC 1/4 at 20 tpi and 5/16 at 18tpi are the same bar the thread angle, I suspect some people sell the latter as both.
Hexes on BSW should fit best into BSW/BSF weapons - though, again, there are close matches across spanner series too. But then, yet again, I have seen hybrids with BS pitches and AF heads - but no idea where such things have come from. My A had several head bolts like that when I first got it. Tis a minefield!