The block (1) comprises a shaped body (3) made of plastics material suitable for being fitted on and restrained astride an edge (4) of the window pane (2) and having a plurality of shaped portions (5-7, 10, 11) which bear against corresponding portions of the surface of the window pane (2) when the block (1) is in the mounted condition. Recesses or chambers (12-16) operatively facing the window pane (2) and suitable for housing a quantity of a lubricating material are formed in the surfaces of contact with the window pane (2) of at least some of the shaped portions (5-7, 10, 11).