1979 A1 Carburator Metal Disc Measurements

JIC this might help someone else some other time down the line....026-01-09 Kz1300 Carb Disc Dimensions

Several of my carbs have the metal disc replaced with plastic discs..Here's OEM metal disc measurements.

 
 
 
 
A 1/4" bit fits the hole ID almost perfectly
 
 

For additional details like weight, etc. from 'Oz' at Zed-Parts
clicky here

The plastic diaphragm plate (you show the OEM metal ones) is from a "Dyno Jet" Stage 3(?)  KIT. 
The lighter weight of the plastic holder provides a faster response in terms of Vacuum lifting the main-needle holders/slides. 
I would advise you abandon the "o-rings" you were intending on using.  A "Fiber" washer" is required........or use nothing at all ! 
"Lock-Tite" will seal the 4 small screws.

I rebuilt a set of carbs for someone several years ago and it was the first time I had seen and ever have seen a "Dyno-Jet" Stage 3 kit.   I wasn't impressed with the "Plastic" retainer-ring.  It was deformed(warped) and lifting between the 4 screws.  A poor choice of materials for the application and the PRI$E ! 

Thanks Scotch... Good info. Passed it on to Oz at Zed-Parts as he has seen the plastic discs too and didn't know where they came from..