  We give a physical notion to all self-adjoint extensions of the operator
$id/dx$ in the finite interval. It appears that these extensions realize
different non-unitary equivalent representations of CCR and are related to the
momentum operator viewed from different inertial systems. This leads to the
generalization of Galilei equivalence principle and gives a new insight into
quantum correspondence rule. It is possible to get transformation laws of wave
function under Galilei transformation for any scalar potential. This
generalizes mass superselection rule. There is also given a new and general
interpretation of a momentum representation of wave function. It appears that
consistent treatment of this problem leads to the time-dependent interactions
and to the abrupt switching-off of the interaction.
