defined by a head end 141 and a connecting end 142 the extension section 140 has the same cross section and is of the same size as the main section 110 which is square in the embodiment being described and likewise has a hollow interior along the left and right outer side walls 143 amp 144 respectively the guide rail extension section has mirroring rules 145 which continue the incremental markings found on the main section rules 115 the inner cavity 151 extending from head end 141 to connecting end 142 is defined by top 146 bottom 147 left 148 and right 149 inner walls the connecting end 112 of main section 110 and the connecting end 142 of extension section 140 are complementarily machined such that when positioned against each other the centerline of the extension section 140 is positioned along the same line as the main section 110 in the embodiment being described the connecting ends 112 and 142 are respectively machined perpendicular to the centerline of main section 110 and extension section 140 it is however recognized that other shapes and angles can be provided within the spirit of the invention as shown fig3 amp 4 an expansion fork 150 has a base 152 defined by sides 153 amp 155 top 154 and bottom 156 the expansion fork 150 is dimensioned such that base 152 fits into the cavity in extension section 140 at connecting end 142 extending outwardly from fork base 152 are upper and lower fingers 164 and 166 respectively dimensionally the same in combination as.