Provided always and be
it enacted that if it shall so happen that any house, wall or other
substantial erection shall be found to be correctly placed at the corner of
any block of allotments so that it is impracticable to erect in that position
one of such posts as aforesaid, then in that case it shall be sufficient to
mark or otherwise affix the said letters in such permanent manner as may be
deemed advisable in their proper places in the manner aforesaid upon such
house, wall or other substantial erection.