These are excerpts from Le Carré's Tinker, Tailor, Soldier, Spy:
Jerry Westerby screwed up his face in perplexity. 'That's what the boy wanted to tell me, you see, George. That's what he was trying to put over in Stan's bar. What all the rumours were about. The Russians moved in on Friday. They didn't shoot Hajek till Saturday. So the wise lads were saying: there you are, Russians were waiting for Hajek to turn up. Knew he was coming. Knew the lot. Lay in wait. Bad story, you see. Bad for our reputation, see what I mean? Bad for big chief. Bad for tribe. How.'
'How,' said Smiley, into his beer.
... and later on,
'Any time,' said Westerby again. 'Sorry, you know. Love to Ann.' He hesitated.
'Come on, Jerry, out with it,' said Smiley.
'Toby had some story about her. I told him to stuff it up his shirt front. Nothing to it, is there?'
'Thanks, Jerry. So long. How.'
'I knew there wasn't,' said Jerry, very pleased, and lifting his finger to denote the feather, padded off into his own reserves.
So, how does one interpret all this how-how-ing? What's the difference between an echoed how and one which is not? Is it a peculiarity of the English public school system? If so, is it still in vogue today?