Yes, I agree the curve "up to 300F" is conservative taken, neglecting the variations of properties (E and limit of stress-strain elastic domain) for temperature less than 300F, so you can call the curve "design values for temperature up to 300F".

Probably, in the same degree of approximation, for cases when "A" is to the left of graph and temperature is less than 300F, the modulus of elasticity may be E=29*10^6 psi as is written on graph (despite there are accurate "E" values in Table TM-1 of ASME II D), and one would consider that using more accurate values for "E" is not really an improvement. But, as you said, literature is silent also on this issue.

By the way, for such cases ("A" is to the left of graph) is strange (but not wrong) that the Table CS-2 and Figure CS-2 starts in different points. Table says A=0.176*10^(-4) and B=250 psi and graph starts with 0.000176 and 2500 psi.
Both pairs are "correct" (in the limit of work approximation), just graph is more truncated than the table.