KUY12 
<p>Vintage High Power Silicon
NPN, 210V, 10A, 70W, 9MHz, TO3 
</p><p>Other markings: "S20 321" (date code S2 - manufactured Feb 1984)
</p>
Made in  former Czechoslovakia by Tesla.
<a href="//commons.wikimedia.org/wiki/User:Mister_rf" title="User:Mister rf">Mister rf</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>