I don't have a series vi to play with. But for a series vii a 49mm to 55mm step up ring works great as an adapter. The 49mm thread centers in the clamping ring of the series vii, so you just epoxy the two together. This lets you mount a 55mm threaded filter onto the clamping ring and screw that onto the series vii mount.
In the same way, a 30mm to 37mm step up ring works on a series v. Somewhere between those two there should be a thread size that fits right in a series vi.