When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Nobody really knows for sure what the very last segment is, but yes the date code is the 18 J portion.
Some thought it might be the number of that model, others the count of that model year to date, others the count of that model that month.... none of em really make sense.
For example, mine is a stakebed and has the number 6. Other stakebeds don't necessarily have a 6. Being a July truck I seriously doubt it was the 6th stakebed of the year, but I might be convinced it was the 6th stakebed that month. Some people have 5 digit numbers which would sound like an incredibly busy month to put out that many of any 1 model in a month.
I have junk yard access to a number of KC trucks, and if we suggest that each plant used their own code scheme it still doesn't make sense any of the proposed decodes with those few samples.
I'll add that Bryan has a pretty good handle on the answers. You know about all there is to know regarding your truck from tag data. The answer to the question about the last numbers of the production code is lost to time. But since you have the later style tag, introduced toward the end of 1951 production, it's fair to say your truck was scheduled for build in September, 1951. I'm guessing the consecutive unit number in your vin is probably pretty high.