DV,
I don't think this is an RAF seat, but it looks similar enough to illustrate how the drogue shackle sits on a modified seat:
I see what you mean about the 'lug' not being affected by the tightness of the bolt - it doesn't appear to be a collar-type affair. I agree that it's difficult to see how the shackle might improve low altitude performance - about the best speculation I can offer is that it
might have come as an essential part of the gas bladder improvement. Other than that, it would appear to be somewhat distinct from the boarding mass issue.