The ornamental design for a letterrack, as shown and described.
FIG. 1 is a front left perspective view of a letter rack showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is an additional front left perspective view thereof; and,
FIG. 8 is an additional front left perspective view thereof.
The broken-line disclosure in the views is understood to represent portions of the article in which the claimed design is embodied, but which form no part of the claimed design.