looking through the 3dr chassis numbers I have and there build dates, it looks like the end numbers are monthly batch numbers rather than an accumulating tally of 3drs, so when they fall into the next build month one of the middle letters of the VIN changes to signify this and the end numbers drop down again

From what I understand the 3drs were built alongside other mainstraim sierras so the end numbers are a total tally of sierras rather than just 3drs for the month

The last batch of 3drs were assembled in Dec 86 and from feedback I have received some went straight on d plates in early 87, where as all the e platers so far were built a month or two earlier but then not registered until August 87

So to answer your question the last one would have a December build but its VIN will be mixed up with a load of other sierra versions so not easy to find unlike the way they numbered the 500s