A GM mistake on the 1947 Advance Design cab is the lack of a water trough on the top of the cowl. Heavy rains allowed water to run under the hood and down the firewall. No doubt some water damage would occur to the voltage regulator and the cloth covered wiring harness.
By 1948 GM corrected this problem by adding a side to side trough in the cowl. The photos show cowls with and without this water trough.
It should be noted that this trough is still lacking during the 1948 in slower selling cabs such as the Suburban and Panel Truck. Possibly these bodies were produced a year ahead.
1947 no water troughs (above)
1948-1955 water troughs in place (above)