40% seems correct:

“Once it was apparent that the situation was manageable and that the underlying business was unaffected, the AMEX stock started to recover. AMEX was a big position for the Buffett Partnership at once the stock started to appreciate it grew to 40% of the total portfolio. As Buffett was proven right, he started exited the AMEX position. It had taken five years to play out.

On January 24, 1968, Buffett wrote to his shareholders: “Last year I referred to one investment which substantially outperformed the general market in 1964, 1965 and 1966 and because of its size (the largest proportion we have ever had in anything – we hit our 40% limit) had a very material impact on our overall results and, even more so, this category. This excellent performance continued throughout 1967 and a large portion of total gain was again accounted for by this single security. Our holdings of this security have been very substantially reduced and we have nothing in this group remotely approaching the size or potential which formerly existed in this investment.”

fundamentalfinanceplaybook.com - The salad oil scandal of