Tom, Your 1949 Panel "serial number" breaks down as:
5 = Kansas City Assembly Plant
GP = 1949 3100 Series (1/2 ton chassis)
E = May (month assembled)
18016 = 17016th truck assembled at the Kansas City plant for that year (the first truck assembled started with serial# 1001)
This information was found using the
1949 Chevrolet Truck Data available from the GM Heritage website. A screenshot of the 1949 serial number info is attached below, but is kinda hard to read. I attached a snap of the 1948 serial number sample info as it is virtually the same as 1949, just easier to read.