2SK1234 == K1234 / 2SJ1234 == j1234 always ?

:ignoring the number part, only for depiction here :like 2SA.... = A..... etc, just abbreviated forms : :Is there an earlier series where J123 meant J123 and different to a later :2SJ123 ? : : :

Siliconix (now Vishay) start some of their J-Fet devices with just a 'J'. ie.

Yes, I was going to say something similar. J-FETs with just a "J" and no other 'missing' letters or numbers, are found quite commonly in group amplifiers. Given that you repair quite a lot of these, I'm surprised you haven't come across them. There's one amp (and which it is completely eludes me at the moment, but it's something that I see quite often) has a couple of these slung across signal lines as 'shorting switches', and they regularly go leaky or short, giving low or no audio.

Arfa

IIRC Hitachi did the same thing with their TO3 fets like the J49/K134 J50/K135 etc...