Overbore A5 mounting during WW1

[Marksman]

On a Ross Mk III set up for a Winchester A5, where exactly is the rear dovetail block fitted?

If on the receiver ring, is it entirely on the larger rear part (nom. 1.5" dia), or partially on that rear part and partially on the smaller front part (nom.1.4" dia) with the dovetail block having a stepped base?

Or is it partially on the front part of the receiver ring and partially on the cylindrical rear part of the barrel reinforce (nom. 1.3" dia), again using a dovetail block with a stepped base?

Or is it partially on the cylindrical part of the barrel reinforce and partially on the tapered part, using a dovetail block with a base with a flat and a taper?

None of the contemporaneous photos that I've found are sufficiently detailed, but I get the impression that the rear block is mounted in front of the larger diameter part of the receiver ring.
Hope someone can help.