Nightbirds have something similar:
Might even be the same ones, or the stewmac version a bit more evolved.StewMac has those:
Gripper Strap Buttons, Set of 2 - StewMac
These extra-wide strap buttons will keep your strap from accidentally slipping off your guitar.www.stewmac.com
Ralf
They look a bit different I think. Good to know the old Nightbird strap buttons break when getting old.Might even be the same ones, or the stewmac version a bit more evolved.