As stated earlier, he (and any other convicted felon) can legally go out and buy pre-1899 firearms, because they are specifically defined as non-firearms under the '68 GCA. For that reason, anyone can buy such weapons without a NICS check, wouldn't have to go through an FFL, and wouldn't fill out a 4473. Oh, and only the receiver needs to have been manufactured pre-1899 - you can get a new barrel, trigger job, etc. if that's what you want to do. The only problem is that the number of such guns in decent shape in calibers that still have ammo produced for them is quite limited - but they are out there.
It wouldn't surprise me to learn that "Mrs. Liddy" also owns quite a number of Class 3 weapons. I would doubt that any home invader short of the entire 82nd Airborne Division would get out of their house with a detectable heartbeat.
Liddy is the kind of guy that makes me believe that our system is utterly unjust - he did commit a crime (several, actually), and he properly served time and went through parole as punishment - in other words, he paid his debt to society. He is no longer a physical danger to anyone - if he ever was - who doesn't first attack him or his family. He is now reformed - he hasn't (to my knowledge) committed any crimes since those he was convicted of, and he engages in a productive activity that supports him and his family, to say nothing of putting lots of dollars into various governments' coffers. He seems to be the ideal ex-con, yet our system says that this man has no right to defend himself against an attacker with the most effect means available. While I wouldn't want to take him on if I had a machine gun and he was unarmed now, what happens when he's 80 and (relative to his condition now) feeble - what's he supposed to do if some 24-year-old guy who weighs 260 pounds attacks him or his family? The system sucks, and for Liddy's sake I hope that his wife outlives him, so that he'll always have guns "stored" nearby.