The ornamental design for a helicalpost having a slotted mounting base, as shown.
FIG. 1 is a top perspective view of a helical post having a slotted mounting base showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a perspective view thereof.
The broken lines shown in FIG. 9 which show the bolts and the pole above the helical post having a slotted mounting base illustrate environment and form no part of the claimed design.