& the adapter + FD has been allowing the inter face to rotate the parts relative to each other (done with wedges between the Parts)

Of benefit to Mfgr, make 1 of the relatively expensive piece & several sizes of band clamps...



I have 1 FD adapter made to clamp on the Brompton seat post, custom brazed together, Made to compensate for the seat post passing behind the BB
rather than aligned directly into it, in bikes that dont fold, telescoping the seat post .

simpler to make that piece, than a whole new FD type, for a small shop.