The Napco emblem shown is now sold as 57-59. Those were the "factory years". This indicates that the Napco kit may not have come with an emblem in pre 57 kits. The 3600 emblem is 53-54 as said. The first J in the serial is from a different stamp set. The reason the installs were all the same is they followed the instructions.
Plenty of units could have been self installed to save money. Many in those days were purchased by utility companies, government, oil, gas exploration, miners, loggers. They would have had their own maintenance shops.
Other buyers were farmers and ranchers. They were better equipped and they were better mechanics than anyone else.
Very few were bought by a hunter or a city slicker. You had to have a real need to spend the money....or be wealthy.
One thing I can tell you for sure is that the vendors do not all know what it is they are selling. One person reproduces an item and then sells it to other vendors. Does not mean they all label the applications correctly. I remember when the reproduction badges were first being tooled up and as I recall it was Jim Carter that had it done. NAPCO POWR-PAK badges came with all upfit truck kits from 1953, when it was introduced, clear into the late 1970's when they stopped doing upfits on larger trucks of many different makes. They are literally the exact same badge for all those years. Conversely, from June of 1956, when the front driving axle RPO became available from GMC 100, 150, and 250 series trucks, through the end of the 1959 model year, factory installed 4WD never included a NAPCO fender badge. GM was not in the business of marketing NAPCO, they marketed their RPO when it came to factory jobs. They are not unique to 1957-59. That is not an accurate application listing. You'll also see transfer case shift pattern plates and warning tags that are specifically for twin-stick Timken T-32 transfer cases, which were used in upfits only, from 1953 to late 1955 and very very frew 1956's, being sold as correct for all NAPCO trucks. I see 'restored' 57-59 trucks with both the Spicer 23 shift pattern sticker, and the Timken T-32 dash plate installed in the same truck. One pattern for a single stick t-case, and the other for a twin stick t-case, because vendors have them marked for the wrong years, and mention nothing about which transfer case they pertain to.
The first J indicates that it's a 3600, and is consistent with the GVW. I have seen plenty of door post tags where the first letter is not from the same stamp set. Not that uncommon. This is my first truck out of Janesville, though, so I'm not sure if it's common from that line or not.
