ZTTO levers are decent and can be had for under 20 quid from ebay.
Do they mount to I spec B?
No idea.
https://www.ebay.co.uk/itm/133007098951
https://www.singletrackbikes.co.uk/m7b214s430p55913/Pro_Koryak_Adjustable_Seatpost_LH_shift_lever_assembly_I-spec_II_and_clamp_on_/Saddles/RS_GB
Edit - never used one, but piqued my interest and is the right price for a dropper lever.
Edit X2 - Might also have the issue described above whereby it's designed to have the cable threaded the other way.
@Dogs started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
ZTTO levers are decent and can be had for under 20 quid from ebay.