I'm sorry to hear of Mort Caplin's death, although it's great that he made it to age 103 (and was mentally sharp even when extremely old).
I worked at Caplin & Drysdale in the early 1980s, and I agree with Scott Michel (who started the same year I did but stayed a lot longer, and is quoted in the above article) that Mort set a crucial positive tone regarding the office atmosphere, as well as one about serving clients assertively if needed but ethically. He was also affable and charming, including to junior associates.
Only on one occasion did I work with Mort directly on a project, but it left a good taste. He was interested in a live issue at the time, concerning the IRS's ability in audits to access tax accrual workpapers that a given taxpayer's accountants had used to evaluate tax risks for financial accounting purposes.
Mort was thinking about pursuing in litigation a legal position to the effect that the IRS should generally be denied access to such workpapers, on the view that allowing it would undermine financial accounting by inducing accountants (or taxpayers in discussions with them) to pull punches regarding soft spots, the taxpayer's possible settlement strategy, etc.
This concern is clearly a meaningful one, even if on balance one nonetheless favors granting access. But my task (obviously) wasn't to figure out what I thought about the merits, but rather to look at precedents, etc., for a sense of how strong the case would be. I concluded from my research that, based on the case authorities etc. to date, the IRS was extremely likely to win on this issue. (As indeed was soon confirmed - see U.S. v. Arthur Young, decided by a 9-0 Supreme Court in 1984.)
After working all day on a Saturday or Sunday to finish the memo, I per instructions took a taxi up to Mort's rather nice home (in NW Washington, I think? - but I could be mistaken) to hand him the memo and briefly discuss it. Given the circumstances, I was grungily dressed and unshaven, and I recall having a harder time getting a cab to stop for me than would usually have been the case when I was wearing my regular weekday suit and tie.
When I got there, Mort, though gracious, was clearly not pleased with my conclusion, as it was not what he wanted to hear. But I got the sense that he accepted both its legitimacy (although obviously he didn't read it in detail until I had left), and that I had properly done my job.