It can get a bit confusing, but If the model is a Blue Streak, regardless if made by Sheridan, Benjamin or even the later Crosman made version, it's a 20 cal. That 20 cal is the trademark of the Blue Streak or Silver Streak model. Any other caliber is a Benjamin or Crosman model.
The Benjamin made Blue Streaks are still the same model as the earlier Sheridan made Blue Streaks because Benjamin continued to make Blue Streaks in Sheridan's original Racine plant after they bought out Sheridan in the 70s.
When Crosman bought out Benjamin/Sheridan in the early 90s, Crosman moved production to E. Bloomfield and made some changes to the Blue Streak design. These Blue Streaks are the C9A Blue Streaks. Still 20 cal, though.
Regardless of caliber or model, any of the walnut stock Sheridan or Benjamin or Crosman pump ups are great guns. Even the last of the walnut stock versions made in E. Bloomfield by Crosman are excellent. I have a mid 90s walnut 397PA made in E. Bloomfield, the last of the walnut 397s that is one of the most accurate pumpers in our collection of Benjamin and Sheridans. These are models with the tang safety, before the safety moved to the trigger guard.
I would not discount the later Benjamin pumps made by Crosman with the hardwood stocks and the safety in the trigger guard, though. These are not as collectible as the walnut stock versions, but they can still be excellent shooters.