import os
import arcpy
workspace = r"C:UserslawtondRastersWoody"

walk = arcpy.da.Walk(workspace, type="GRID")
output_location  = r"C:UserslawtondRastersWoody"

for dirpath, dirnames, filenames in walk:
    for file in filenames:
        if "woody13" in file:
            list_raster.append(file)

print (list_raster)

arcpy.MosaicToNewRaster_management (list_raster, output_location, "merged_woody_2013.tif","1", "FIRST")

Message File Name   Line    Position
Traceback
    <module>    <module3>   18
    MosaicToNewRaster   C:Program Files (x86)ArcGISDesktop10.3ArcPyarcpymanagement.py 13191
ExecuteError: ERROR 000622: Failed to execute (Mosaic To New Raster). Parameters are not valid.
ERROR 000628: Cannot set input into parameter coordinate_system_for_the_raster.