This tube was rebranded by International Components Corp. Most older metal tubes have the type number stamped into the base like the "U.S.A." that shows up clearly in the picture above on the left. I became suspicious when I realized that the type number had been filed off or ground off these and the tubes were repainted. As it turns out, the stamping process leaves a mark on the inside of the metal skirt. I removed the base from one of these, and indeed "12SJ7" shows up clearly as shown in the second picture above. "12SJ7" was reversed on the inside of the skirt, so I reversed the image for easier reading. The 12SJ7 is a sharp cutoff pentode, while the 12SK7 is a remote cutoff pentode. The 12SJ7 and 12SK7 test similarly on a tube tester and will work for each other in many applications. However, if the brander was not afraid of saying this, why did they grind off the number on the tube? The answer is that most applications calling for a 12SK7 are optimized for a 12SK7. So, a 12SJ7 will give degraded performance.