JW.. .Is this the hub you need to dismantle, or have you had it apart, reassembled it at it won't fit? Is it another hub entirely?
Looks to be a plunger hub, with the two bearing retainers. To my estimation they both need to be further into the hub, the right side one is especially proud and will prevent the shell cover seating. So, could be wrong bearings, or bearings not seated, wrong centre bearing spacer.
The centre spacer is a simple tube, with two pressed on discs, which have cut outs to pass the hub rivets on assembly. The design is such that the wheel spindle passes through the sprocket drum right through the bearing sleeve and its large retaining nut, then being tightened with its own stepped nut. You need to check you have the right parts and they are made to correct dimensions.
The brake back plate lip runs in a groove in the sprocket drum, the clearance here should be just a couple of mm at most, and the problem could be in the assembly of the drum/back plate. Perhaps a now unnecessary spacer to remedy some previous bodge. Spurious parts and poorly made pattern parts are all to be expected. The answer to this will be simple, so have a look at internet images of these parts, see how they compare.
Anyone got one in bits to hand for a bit of measuring?
Swarfy.