> What if, when the model reads a file, or greps for something, every line comes back tagged with a 2-3 character content hash

So, that’s what columns 73 to 80 are meant for!

https://en.wikipedia.org/wiki/Fortran#Fixed_layout_and_punch...:

“Columns 73 to 80 could therefore be used for identification information, such as punching a sequence number or text, which could be used to re-order cards if a stack of cards was dropped; though in practice this was reserved for stable, production programs”