It may refer to the passing of Ernie Davis who died in 63.Brown made a trade a year or two earlier with Washington I believe, to secure Davis in the draft. He was subsequently diagnosed with leukemia and never played a NFL game.
The doctors said Davis could play but Brown wouldn't play him until he got better, and he took some heat for that.
I don't know how Shaughnessy or his tax return fits in to the mix. As coaches I am sure they knew each other but I don't know if they liked each other of if Browns displeasure is in reference to the letter Shaughnessy wrote.
But, that is just what I recall...the letter could refer to many things
Thanks for the input. The Brown letter to Shaughnessy is to thank him for his support and good wishes. He also said something about Shaughnessy 'having gone through it" himself (maybe another tragedy involving the Bears?). The tax return may be something separate--just part of Shaughnessy's old papers I guess.