I have checked another spider and the measurement of 19mm is correct.
I did wonder if you had fitted the bottom plate incorrectly (one finger offset, if that makes sense), so that the bottom plate wouldn't sink to the bottom of the basket. I even thought that perhaps you had put it together correctly on the first photo (the one without springs) but incorrectly on the second photo. Having said that, your measurements seem to support the idea that this in an incorrectly manufactured item.
Hmmm. Incorrectly manufactured or not, I'm sure there is someone who would like to get their hands on such an item so they can made a seven plate clutch!!
Do you need to bend the ear up on the bottom smooth plate? It can slip down below the spider otherwise.
One thing before I hit my head off the wall one more time...
The flange doesn't seem to sit properly. There's a little bit of wobble in it with the X-mas tree centre touching the underside. I'd say at a guess there's about 1-2mm in it.
The Christmas tree (MainShaft) normally hits the bottom until it is tightened up this pulls it up to the end plate and away from the inside of the case...