Assuming all other dimensions on the drawing are in imperial units, I would normally interpret it as a notation for 2" diameter thread, 4" long ("LG"), 8 tpi (threads per inch ["thd"])which would probably be defined by the Unified Constant Pitch Series.
However the term "S.F.Tap Bolt" appears to be an abreviation of Self tapping bolt, and I haven't come across a self tapper this big before !
That's is just another part of the vernacular around the fastener supply houses. I have very seldom heard the term threads per inch used, it is always "give me 20 2"x12" 8 thread, 7 thread, or what ever.
Another quirk is the length callout on fasteners, especially studs. If nothing is said the length will be end to end which takes a little usable thread off due to the chamfer. More and more people were calling out 1st to 1st , the first good thread. Now that material cost has gone through the roof the callouts are back end to end.