Copper usually does not form a heavy lime scale.

Could you have something that broke free from further back in the water supply? Like a chunk of rust. If the house was moved in the 60's and it wasn't completly re-plumbed there may be some galvanized in there somewhere. That would really get a heavy build up.

Do you have a softner? Dosen't sound like it. But if you do and it has failed, the media inside can get loose and plug things up. Toilets and their valves are very susceptible.

If you have the flex line off and running into a bucket, try turning the water off & on, quickly a few times to get a little water hammer going. Might break the obstruction loose and flush it out. First be sure your main valve works.