Many or most readers of this site probably know that most chips (and PCBs) have the date of manufacture stamped on them, almost always indicating the year and week (usually not the actual date) they were made.
Especially with PCBs, there is no standard for whether the year or week comes first. For pre-millenial hardware that’s not a problem, because it’s unambiguous what’s the year and what’s the week. It’s a bit tricker when something like 0403 is stamped on a PCB—is that week 3 of 2004 or week 4 of 2003? In such cases, both might be plausible and one has to look further.
Chips more or less universally use the year/week convention, but some manufacturers label their products with a two-digit year and others only use a single digit. Intel is of course in the latter category. The above image shows a typical vintage Intel chip with a date code stamped on the bottom side of a chip and etched on the top (a 90 MHz Pentium, manufactured in week 15 of 1995). Note that there are actually three date codes, 515 on top, and 501/515 on the bottom of the chip. Intel makes things slightly difficult by often embedding the date code in a longer string, with the three-digit date code starting at the second position (the first might be a digit or a letter).
Most of the time, it’s easy to tell whether a date code like 927 means week 27 of 1979, 1989, 1999, or 2009. But sometimes the answer is not so obvious.
While researching the somewhat unusual Intel 287XL FPU (a 387 math coprocessor in a 287 package), I came across this web page which shows (in the leftmost column) a 287XL with date code 643 and claims the chip was manufactured in 1986. But that’s impossible because although the copyright printed on the chip is 1980 (8087) and 1986 (80387), the 287XL only came on the market in 1990. It is far too difficult to believe that Intel would have secretly started manufacturing the chips 4 years earlier.
But that only leaves one possibility which is only slightly less strange: The 287XL was manufactured towards the end of 1996. Did Intel really still manufacture 287XLs next to Pentium Pros? The 643 date code certainly suggests that…
Unfortunately, while the introduction of a chip is usually easy to pin down, the date when the production was stopped may be impossible to establish. It’s not something that’s usually announced very loudly, since by definition no one cares much about the chip at that point.
Given that the 287XL was an aftermarket upgrade, it’s quite possible that Intel manufactured the chips for longer than one might expect. Most images of 287XLs available on the Internet show chips manufactured between 1990 and 1993 (again, if Intel started the production in 1986, where are the 1987-1989 chips?).
This page shows a 287XL with the same date code and overall the same markings as the one linked to above, but based on the lack of scuff marks, it must be a different specimen. The site likewise incorrectly claims the FPU is from 1986.
Maybe there was a production run of the 287XL in 1996 because someone wanted to upgrade a bunch of old computers? Or the 287XL was still slowly but steadily selling, so Intel decided to make a few more? Either of those explanations is far more likely than Intel making the chips in 1986 and only selling them years later.