I think you are actually asking about the booking class, not the fare basis. A fare basis is generally 6-8 characters; for example, PLX57SNC. The first letter of the fare basis is generally the booking class, and as you say, LX uses P/Z/D/C/J for business class. Are you searching on Swiss' website? If so, there is an option to "show booking classes" when you are selecting which business class product (basis, plus, etc.) you want.