The majority of the IBM model M keyboards use a canned crystal clock of two pins like this one 
, and can be seen throughout the following pictures:
(ref:https://mechanicalkeyboards.com/)
vs. the 4Mhz square quartz crystal of 4 pins on this keyboard, that cannot be unseen:
ABOUT THE CRYSTAL:
Millions of IBM model M keyboards were manufactured, and this is the first one discovered of only hundred out there maybe. On record, there's actually just a couple dozen publicly known models to have it. Do your research.
Very early manufacture date models and development keyboards are among the ones discussed to have used the square crystal oscillator.
To put some perspective on the issue this keyboard is even rarer than SSK, Industrial and M15 put together.
Only one keyboard with the rare square crystal oscillator is on sale. That's it.
MORE INFO:
K/B P/N: Unknown. No back label.
Controller PN-1392993 EC-528154 6523 55C (pic 8)
U1 / U3 P/N:#2396191
U2 P/N:#1389986
Y1: 1351544 4.00MHz TEW5 43 A (Made by Tew denpa co. ltd. Tokyo, JAPAN. Outer shell is transparent.)
Made in United Kingdom
PBT keycaps with thick labels.
Motorola 6805 IC
Top casing date stamped Sept 1985. (pic 6)
Steel plate label # 1386717 19-06-86 (pic. 7)
Made in IBM's Greenock factory, Scotland.
Silver Label.
102 keys Spanish ISO.
Tested and working 100% on January 2014, however as this item is 27 years old, I cannot offer return or guarantees.
SDL cable included.
What you see is what you get. If you're unsure of what you're buying, this item is not for you.
Shipping:
Free snail speed worldwide shipping.
If you want a faster method, ask me about it before bidding to get the appropriate quote to your country.
oQ~~60_12.JPG)
