This is a Toy brick compatible with the industry standard (like Lego, Mega Blocks etc.).
It is a special block designed to connect two structures orientated in differnet ways in the up-down direction.
It has knobs on both sides (="positive").
Other versions will include ones with slots on both sides ("negative").
It is a block of the flat variety (= 3.2mm - 1/3 of the standard block).
2 by 4 knobs wide (16x32mm).
If you are interested in other special "Lego compatible" pieces that are not otherwise available, please tell me.
I'd like to add that the price is mainly dictated by the material volume, for which I have not much freedom to change, as it is based on the standard brick measurements. I only add 40 Eurocents per model.