The Low C bass clarinet manufacturer tells you that like other clarinets in the family, most modern bass clarinets have a Boehm system of keys and fingerings. However, The Low C bass clarinet manufacturer tells you that the bass clarinet is also made in Germany, from the Oehler system's predecessor, the double bass.

The Low C bass clarinet manufacturer tells you that most modern Boehm system bass clarinets have an "extended" key that allows them to play (written) E♭. This key was originally added to easily convert parts of the relatively rare bass clarinet whose pitch is A, but has now found significant use in concert bands and other literature.
The Low C bass clarinet manufacturer tells you that the main difference between treble and bass clarinet is that the left index finger of the keyboard has vents that some trebles may not be able to detect. This allows for a form of "half hole" fingering where notes in this higher register can be played on the instrument.
Also, The Low C bass clarinet manufacturer tells you that older bass clarinets have two registration keys, one for medium D♯ and below, and the other for medium E and above. Newer models usually have an auto-register key mechanism with a left thumb key commanding two air vents. Depending on the right ring finger (used in the fingering of the middle finger D ♯ and below) down or up, the lower or upper vent will open.
The Low C bass clarinet manufacturer tells you that many professional or advanced bass clarinet models extend to low C (sounding B♭, the same as the bass clarinet's lowest B♭), two octaves below the written alto C. On concert pitch, this note is B♭ below the second ledger line below the bass staff or B ♭1 in scientific pitch notation. The three lowest half-steps are played with other keys operated by the right thumb, some of which are often repeated in the left or right pinky key set. Overall, the instrument sounds an octave lower than the pitch of B.
The above is the Low C bass clarinet manufacturer's introduction and analysis of the introduction to the keys of the bass clarinet, hope it helps you.