G Series - Formally known as BSPF The BSPP Thread Gages conform to to BS EN ISO 228-1 Parts and -2 for Gages. The standard year is 2003.   Screw Thread Plug Gages are to a standard class of fit.  British BSP Thread Ring Gages Screw Thread Ring Gages are available in Class A and Class B Class A on G Series rings is more common and considered standard. Class B only refers to rings, there is no class B on screw plugs The go thread ring gauge is a class A only.Class B does not exist but you may get inquiries asking for a Class B go ring in which case we will mark the Class A as a class B. The difference is only on the No go ring. Class A is a tighter tolerance than a Class B, The tolerance on a class B is twice that of the Class A. In March 2003 the British Standards Institute withdrew BS-2799 and replaced it with ISO-228 ( technically BS-EN-ISO-228 ) Full Form British Thread Gages sold by Thread Check Inc are full form gages traceable to U.K.A.S https://www.threadcheck.com/g1-8-x-28-bspp-go-no-go-thread-ring-gage-set/thread-gages-gauges/