Hi,

https://www.krale-schietsport.nl/en/brands/optisan.html

Optsain Champion 30mm low mounts

These are the lowest ones that I use and have come across, lower than BKL, Sportsmatch and B square,
They are very well made, and some 2mm lower than both BKL & SM, 1mm lower than the B Square.

Good price too and the company are very good to deal with.

ATB
Mike