I believe this was kind of an individual truck thing and probabably varied from assembly plants and also who worked the assembly line and where on what day. My 51 1/2 ton built at Norwood OH. has sbox and the no. 70 in white written on the firewall while my 50 1/2 ton built at Kansas City has no markings at all that I can find. Both have clean, original paint firewalls so I think there pretty accurate.