I'm amazed you didn't have any shorted cathodes. I had that happen on several of mine and I gave up using them, except for an occasional ride in the microwave oven. Also, I'm not a fan of the upside-down-2 used as a '5'.
Have you tried depoisoning at higher current, say 8-10mA ?
Funny thing about the IN-1 is the sockets cost more than the tube !