There is a Chinese company that still sells eyepiece screw-on "sun" filters. They give the warning below (in red).
I think the best warning is "Do not use these filters at all - high risk of cracking and causing serious permanent eye damage!!" I think these type of filters are banned for sale in Australia.
They are very cheap - if you are tempted to buy one - don't.
Note:
Solar filter can only see the sunrise and sunset
Viewing time from 30 seconds to 60 seconds for the gifted
Not long look
Otherwise there is a risk of explosion lenses
Had a friend in the 60's who had and eyepiece "sun" filter, the first time he found the sun he turned to tell his brother, heard a pop...the eyepiece was shattered and that was in seconds and it was only a 3" reflector. Lucky kid but a scary story.
Had a friend in the 60's who had and eyepiece "sun" filter, the first time he found the sun he turned to tell his brother, heard a pop...the eyepiece was shattered and that was in seconds and it was only a 3" reflector. Lucky kid but a scary story.