So looking at the key in online version ‘The Boletes of California’ the last couplet says:
23. Stipe short, often flattened at the apex, colored red in the base and yellow in the apical portion; cap reddish brown
I used the key in the bolete book by Harry Thiers: California Mushrooms.
The X. dryophilus I commonly find in southern California always has red in the stipe, especially near the bottom, and a pinched base. This looks like it has a white base and pinching. What features do you use when you apply this name?
for a typical southern California collection. And
for a common color variant.