Actually it was published in 1961 (Bl. John XXIII having died before '63), and it applies only to advancement of homosexuals to ordination in religious orders. This is the first official Instruction to treat of the problem at the level of the whole priesthood.
Oops. Of course this isn't right, since Bl. John died in 1963. The instruction's still from 1961, though.
Thank God for the return of a conservative Pope with the determination to vigorously address this problem.
In my opinion the 1961 letter applied to the entire Church, even though it was specifically addressed to "Superiors of Religious Communities", "Societies without vows" and (Catholic) "Secular Institutes". It appears that (Catholic) 'Secular Institutes' (such as Catholic Universities and Colleges would cover Seminarian training, as would "Religious Communites", which would also cover many ordinations. The "Socities without vows" covers a wide variety of Catholic monks, lay people such as Third Orders, etc. So it's difficult to understand how this 1961 letter would exclude diocesan Bishops from the barring of ordaining homosexuals.
My supposittion is that the Vatican at the time was particularly concerned by reports of homosexuality within the various religious orders, whom answer to their own Bishops and thus are more independent and more able to covertly express any perverted sexual tendencies, (as the Templars of the middle ages).
But no matter if I surmise correctly or not, Pope Benedict XVI's letter is an extraordinary breath of fresh air.