[firemancjack]

HK used Pachmayer flush mount q.d.sling swivels on the later imported 630 and 770s. I'm not sure if Pachmayer still produces them or not. Best bet is check their website. Millett makes a flush mount swivel set up also but I don't think they will fit into the pachmayers. Check around as I see the Pachmayers at gunshows from time to time.
If it were my 630 and in that good of shape, no way would I install a swivel stud in the stock. That's just me though...