rochler, It doesn't matter 'that they can be used safely'. They are illegal & extremely dangerous. You can consider yourself 'very' lucky you weren't at the E/Pc when that incident took place. You would prob. not have felt any pain, as the retina has no pain sensors afaik. Painless blindness.....
For the newcomer to Astronomy, who do not know any better, please, if you read this thread, DESTROY this type of 'Eyepiece sun filter' ASAP.
L.