You are also seeking a very early date. That will make it much harder to find. Even my earliest 1390131 is almost 2 months newer than that.
On the other hand, if you do get one eventually, there may be a good chance that it will have the luscious old Model F stye label!
Yes, my 1390120 is from 03 march 1986. There were likely only a few thousand made on that day, but it really depends on the plant. It will probably take a year or two of searching to find one. Check in with clickykeybaords and eBay frequently.
I believe this is pre-1391401, so you only have a few model numbers to search, and the most common may be a 122-key (maybe even a model F) or a terminal board. Look for barcode labels, as most of the plants switches over to the newer labels a few months later in 1986.
You can get a sense of scope of how many were made on that day by browsing the model M serial number archive project.
Based on the serial numbers, plant J1 was making about 450 daily 1390131's in may of '86
Plant FL2 was making about 450 daily 1390131's in April of 1986 (not enough data for that plant in May)
for 1390120 Plant J1 was making about 450 daily in March (though it might be as high as 2000 daily)
It's ahrder to tell plant FL2 numbers since the letter at the beginning changes so much, but it seems they might have been making as few as 450 a day or as many as 2000 a day.
In the entire archive, there is only a single KB from your date.
1390120 G0591 5/5/1986 FL2
There were other terminal keyboards and industrial ones that were produced through 1986, but your part number range is quite small. Any 1391401 beige label or blue label is too new. This will help narrow the search.
it will almost certainly have the barcode label the fohat.digs is talking about.
Here's the picture of mine:
