Previously, and see also here.
Proof or not, I totally bet he did. If he didn't, it would have been through spectacular self-restraint, and Walter was not known for that quality.
Incidentally, that's my friend and webmaster substitute in the first panel. We really had that exchange, too. He understands me.
I would say more but I'm juggling five editing projects at the moment and I have already spent far too much time today drawing dead lobotomists when I should be earning money.