I think a single-digit year code can mean 198X instead of 199X in some cases, at least when the unit contains a main chip labeled as "PICO" something. I have pictures and an explanation here,
Thanks,
Dragon! Until recently, I had no 1980s-era modules, but I believe you are correct. I just recently found a couple of brown, BSR brand X10 appliance modules, and until now had no reason to take them apart. But here I go.
Model X10-014601 (appliance 2-pin), module date code 2B06, PICO IC date code 8004
Model X10-014611 (appliance 3-pin), module date code 0K47, PICO IC date code 8042
I think the first is probably 1982. The second could possibly be 1980 since its IC was manufactured 5 weeks prior. I don't know if the BSR brand name was still around in 1990. The month and week line up correctly for all 3 dates.
One thing that has me a little confused is that the second module has a much more contemporary design than the first, suggesting it may be 1990. It has a ratcheting relay, while the first one supposedly made in 1982 has a non-latching open-frame SPST relay and an IC with a wider DIP package. My other thought is that the "0" on the sticker looks a lot more like the letter "O" than a zero and it may be an inspection sticker rather than a date code, i.e. inspected OK by #47.
I'm attaching a photo of the inside of the 1982 BSR X10-014601 just because I've never seen one like this and I am in awe of how antiquated it looks. In contrast to every other X10 module I've seen, it has a very bulky open-frame relay that does not latch and appears cobbled together from 2 discrete parts (a limit switch and separate electromagnet). It's also heavier than the other module and always resets to "OFF" if power is lost. Aside from its vintage aesthetic, it also appears to be much more complex to assemble and mass produce.
The question now is how to determine whether it's 198x or 199x without disassembling the module!
I updated my first post in this thread to reflect your finding and gave you a "helpful" point.